Zachary Pickett was a 16-year-old lifeguard and high school water polo player in Shingle Springs, Calif., when a swimming accident left him paralyzed from the chest down and confined to a wheelchair.

But Pickett, 18, surprised his classmates at Ponderosa High School Friday when he got out of his wheelchair and walked across the stage to accept his high school diploma.

"I was focusing more on the walking part," Pickett told ABC News when asked what it was like to hear his classmates, family and friends erupt in cheers.

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"It was nice to be able to do and nice to be able to get it over with."

Pickett was a lifeguard at a lake looking forward to his junior year of high school when, in August of 2012, he dove into the lake and hit a sandbar, breaking his C7 vertebra, the portion of the spinal column that descends from the skull.

The teenager spent 10 days in the hospital followed by a three-month rehabilitation at the Shriners Hospital for Children in Northern California.

Pickett returned to his high school in the fall of that same year and managed to graduate on time with his class despite his medical challenges. After many months of twice-weekly physical therapy sessions, Pickett took his first steps and set his sights on graduation day.

"Ever since I started seeing some recovery, I knew it would be a good thing to be able to do," he said. "I started with a walker and then have been moving to a cane, just trying to get progressively better at it."

Pickett says doctors told him at the time of his injury that they did not know what his future would hold.

"It's not really like the movies where they say, 'You'll never walk again,'" Pickett said. "They didn't want to say you wouldn't walk again but didn't want to say you would to give you false hope."

Pickett is now looking even farther ahead, to college and his freshman year at the University of Southern California in Los Angeles, a nearly six-hour drive from his Sacramento-area home, where he plans to study business.

Pickett, who has full use of his arms, says he will also continue with his physical therapy to see what else he can accomplish.

"I don't know," he said of his own prognosis. "I know it's definitely going to be a lot more years if I want to be back to normal, which I don't know will ever happen.

"I'm just trying to adapt to what I can do and to strengthen my legs and core to be as physically good as I can be with what I can do."