This high school has one main entrance requirement for its students: They have to be addicts

— -- Walking the halls of the Massachusetts school she oversees, Principal Susan Strong felt something was off with one of her students and decided to confront her.

“You’re high,” Strong said to her student.

“I’m not high right now,” the student shot back. “I’m just pissed, I’m just so f------ pissed.”

“I’m good with pissed,” Strong responded, “I’m not good with high … I’m also good with me being wrong. I would love to be wrong.”

This is a typical interaction for Strong at Rockdale Recovery High School.

Another student named Nicole, 17, said she started doing drugs at age 13.

“I always felt lonely and depressed,” she said. “Like I was a waste of space.”

Matt, 18, used to be on the high school football team, but he says when drugs came into the picture he didn't have time for anything else.

“I did a line of oxy … and I fell in love,” he said. “Every day, I mean, the first thing I think of when I wake up is, ‘How is today going to go? Am I going to use today?’”

For Harry, another student, simply staying alive is a goal he has set. Just 19 years old, he calls himself an “intravenous heroin user.”

“I don’t want to use,” he said. “I just can’t stop.”

Drea, 16, said she used heroin for a year, but is clean now.

“I started to go through withdrawals,” she said. “I would just be depressed and I'd be like, 'I need more.'”

Drea's story:

These students attend Rockdale Recovery High School, an experimental school in Worchester, Massachusetts, with one main entrance requirement: The students have to be addicts to get in. But in order to remain enrolled, the students must be clean of all drugs and alcohol, which is harder for some. Their principal keeps a watchful eye over the roughly 20 students attending, including Matt, Nicole and Harry, all seniors when we met them.

“It’s not like a typical school,” Strong said. “When you’re dealing with life and death every day, the triumphs are the greatest high ever, but the tragedies gut you.”

“I love them like my own children … I don’t want to give up on a single kid,” she added. “They’re going to pick up, use, overdose and die, that’s my greatest fear.”

As the opioid crisis in the United States has ballooned into epidemic proportions and Americans fall victim to opioid addiction at younger and younger ages, many hope the Recovery High School model could be an answer to saving their lives.

The idea is to surround these teenagers with sober friends and classmates, all of whom are dealing with addiction, under the supervision of teachers with an extensive knowledge of addiction to provide a strong support and monitoring system.

“Nightline” spent a year documenting these students’ struggle with sobriety and their principal’s never-ending fight to keep them clean.

This year, Harry isn’t enrolled at Rockdale Recovery High. After almost dying from a heroin overdose in December 2015, he had been in and out or rehab. Strong said it took two shots of the opioid-reversing drug naloxone, commonly referred to by the brand name Narcan, to revive him. He has struggled to get clean ever since.

Harry was clean from heroin for almost 90 days and was free to visit Recovery High for the day. When he walked in, he was met with cheers and hugs from his classmates and teachers.

“It’s bittersweet because we've had these moments with Harry where he's come back and has been in treatment … and without notice … he can be on the street carrying 20 bags of heroin,” Strong said. “There have been too many times with Harry and now I just pray.”At the end of the visit, Harry headed back to his halfway house with hopes of rejoining his classmates full-time soon.

Nicole's story:

Nicole, who had used heroin in the past, said she also has been revived with Narcan before. The worst it ever got for her, she said, was when she watched a friend die from an overdose.

“I felt responsible because I was the one that they used the Narcan on last time,” she said. “I was the reason there was no more Narcan.”

Without drugs, Nicole said she struggles with depression. Strong said she sometimes disconnects from her family. Instead of going home to her family, she often spends her evenings at an apartment where a group of her friends live. Nicole said one of the roommates named LJ is like a mother to her. LJ is also a former heroin user.

“There was a batch of heroin going around that people were literally OD’ing and dying on,” she said. “But when we're in our disease and we're in the thick of it, we don't care. We're going to go for the stuff that's taking everybody out because that's the better high.”They keep a supply of Narcan in the apartment, LJ said, “because you never know when people overdose or relapse.”

“So we don’t have to worry about someone dying from a mistake,” she added.

Matt's story:

Matt said he stopped using oxycontin just two months ago, and is now trying to find his footing in sobriety.

“Mostly what I was escaping from is [that] I didn't feel like I belonged, I didn't feel like I fit in,” he said. “I was more interested in the soothing of the pain.”

Matt said his using has hurt other people in his life. “I remember telling my dad and him just bursting into tears,” he said. “It was horrible.”

Strong thought that Matt needed a “hook”: something that would motivate him and distract him from the urge to use again, and keep him coming back to school. So she added hockey to the curriculum. Matt now says it's his favorite thing to do.

“I don’t know why, it’s just really fun,” he said.

By the end of December 2016, Matt, Nicole and Harry were dreading the approaching holidays. Because for addicts, holidays can be the biggest trigger.

“It’s really stressful to have the whole family together ... usually everyone’s drinking,” Matt said. “I would smoke before going to family events, because it was nice to be high. I know a lot of people who relapse on days like this.”

Harry's story:

But for Harry, this time was especially dangerous. He made the risky decision to leave his halfway house and moved back home with his parents.

“Whenever Harry goes back [home] … he immediately starts using heroin,” Strong said. “And for Harry, that’s a death sentence.”

Fearful he was using again, Susan asked for Harry to visit her. When she saw him, her worst fears were confirmed.

“You looked pretty high right now,” she told him.

Harry admitted to her that he had used Percocet and had smoked marijuana. He denied that he had used heroin this time. Strong decided it would be best if he left his parents’ house and got into a rehab facility.

“Harry doesn't care enough about Harry yet. He doesn't think he's worth saving, and I can't, you can't walk away from that,” Strong said, through tears. “I can’t convince Harry to care about himself, and believe that in 10 years, he’s going to be amazing because he will be. If he’s not dead.”

Strong, who was unable to get Harry to agree to go to rehab, told him she would re-enroll him at Rockdale Recovery High. She reminded him that he couldn’t show up high.

“Well, no s---,” he said.

The next day, Strong welcomed Harry back at school with a big hug. For Matt, the shock of seeing Harry back at Rockdale made him realize that his friend had relapsed.

“Was that a smart choice to leave [the halfway house]?” Matt asked him.

“No, not at all,” Harry said.

Matt then asked Harry what he had been using and Harry admitted he was using Percocet.

“Dude, you’re going to die,” Matt told him. “Isn’t this s--- getting old?”

Later Matt said, “It really makes me sad about Harry because I'm good friends with him and you know he will die, 100 percent. I know it, and it's going to f------ suck.”

The next day, Harry didn’t show up for school. Strong called his mother to check on him and learned that he was staying home that day.

“It’s the worst decision on the planet,” Strong said.

Without being surrounded by his sober friends, and without the close monitoring and attention he gets at school, Strong is convinced Harry will use heroin again.

"I’m so angry,” Strong said. “What are we going to do? We’re going to focus on the kids that are here and be positive because they deserve our attention.”

As the school closed for the holiday break and the students didn’t have classes, Matt and Nicole used shopping to cope with holiday stress. But Harry was struggling.

“So it’s Dec. 27, 2016, a year ago yesterday actually is when I overdosed,” he said. “Sadly, I haven’t stayed clean.”

As Harry drove his car, he revealed that he had been lying to the school. He had been getting high every day on heroin. He then proceeded to buy heroin and smoke it.

“I don’t feel good doing this … I don’t want to use,” he said. “I just can’t stop … why isn’t my willpower strong enough … Is it because I’m weak? Is it because I’m not a man?”

As Harry wrestled with the power and despair of his addiction, Matt had been keeping up with his sobriety and hockey.

“It’s like a natural high and now I have a chance to put everything I have into a sport and not have to think about going home and using,” he said.

Nicole, who seemed a little lost during the holidays, had set a new goal for herself to become a model. Strong had long told her she could be a model, and now that dream didn’t seem so crazy. Nicole and her mother took a road trip down to Virginia for a modeling event.

“I placed first runner-up in my division and I had over 30 callbacks and I say that’s pretty good,” she said. “It was a lot more than I expected.”

But for seniors Matt and Nicole, a new stressor was looming ahead of them: Graduation.

“I’m worried about not having a sense of direction because I think people did that a lot for us here,” Matt said.

But one person who wasn’t at graduation with them was Harry.

After he started using heroin again, his parents sent him to involuntary rehab. After going through a detox program, he then went to a transitional home. He said he wants a future where he lives drug-free.

“I'm doing this to become a better person,” he said. “I want to be able to have the wife. I want to be able have the kids. I want to have a dog with a yard and a white picket fence. I know it's a little crazy but, why does it have to be crazy? Why can't I do that?”

He admitted he still worries that his family and friends will be attending his funeral one day, “but I want it to be a party,” he said.

Graduation day finally arrived at Rockdale Recovery High, and the mood was somber. Matt, in particular, seemed to be acting strange.“It’s like the little bird leaving the nest,” he said.

“I think he's really emotional about graduating,” Strong said, “ but I also know that he relapsed and he's really embarrassed about that.”He admitted that he had a setback recently and smoked marijuana.

“I felt guilty and I was upset about it … but you know I bounced back,” he said. “I am sober. I had to restart the only thing I changed was my clean date.”

On this day, Nicole was proud of herself. She said she didn’t think she would graduate high school, and now a modeling career could be in her future. She said she had an interview with a modeling agent on her birthday.

When asked if she was worried about the numerous temptations that are often found in the modeling world, Nicole said, “This world we live in is a huge temptation… you can’t shelter yourself cause that’s not going to work.”

In their cap and gowns, Nicole and Matt took the walk they never thought they would. They got their diplomas and gave Strong big hugs.

“I've been attending Rockdale Recovery High School for two years,” Matt said. “It has been a life-changing experience, maybe even a lifesaving experience.”

“Moving forward, this will stay with me,” Nicole added. “There are actually good people in this world.”

The road to lifelong sobriety is long and hard, and the students’ principal will have a new crop of teens struggling with addiction next year, but she remains hopeful that her new graduates will have better futures.

“I'm going to miss them,” she said. “They're going to do it, they're going to be rock stars.”

ABC News' Geoff Martz, Lindsey Jacobson and Lauren Effron contributed to this report