"It's pretty random and sometimes pretty monotonous," Garstat said.
A 38-year-old father of three, Garstat works as a radar technician at a nearby airport. He said volunteering for Arpaio's posse is like a hobby for him.
"It gives me the ability to keep my community safe, to keep my kids safe," he said. "Some people take up pottery. Some people take up poker playing. I choose to do this."
Garstat and the other volunteers never actually enter school grounds. They just patrol the surrounding areas, which include strip malls, parking lots and apartment complexes, looking for that dangerous needle in a haystack.
"All of these businesses and communities are interconnected," Garstat said. "The school patrol is essentially a community patrol."
But some gun control advocates said armed civilians patrolling anywhere near a school could be a recipe for disaster. Some of the parents who spoke to "Nightline" at a school on the posse's watch agreed.
"You have people that have weapons on them, that can murder other people, and they are volunteers," said Donna Wetzel, whose son attends Paradise Honors High School. "They are not police officers, they are not military folks and, maybe, they may not like the look of a kid walking down the street. And I worry about instances like what happened in Florida with Treyvon Martin."
While some critics of Arpaio's program have said the posse volunteers are "rent-a-cop" amateurs who could be potentially risking kids' lives, Arpaio stood by his posse.
"I've been doing this for 20 years. Nobody complained," he said. "I used the posse to go after illegal immigrants, dope peddlers, everything else."
Arpaio added that the volunteers receive law enforcement training, but when asked if it was the same training they would get at a police academy, the sheriff said, "not exactly."
"They don't go through that many hours, but they have preliminary training" Arpaio said. "You can go through all the training you want, but it's the common sense that makes a difference."
Arpaio has built his posse over the years and assigned them all sorts of tasks, including patrolling malls during the holidays, investigating President Obama's birth certificate and hunting down undocumented immigrants.
The Obama administration's Justice Department filed a lawsuit, accusing Arpaio of "a pattern of unlawful discrimination."
Activists called it a "reign of terror," and worried that Arpaio's school posse will be part of it, feeding on the pressure from an anti-immigration sheriff.
But Arpaio called their claims "ridiculous."
"We're not out there to check kids or papers of the parents," he said.
Everybody in Maricopa County seems to know Sheriff Joe. One Hispanic mother who pulled up alongside us as "Nightline" rode along with him thanked him for jailing and straightening out her wayward sons.
At 80 years old, Arpaio still likes all the attention he gets. But is his school patrol posse just PR, or a real solution? Perhaps it's both.
"I want this publicity because I want everybody to know we're out there, our armed posse, the same automobile markings as my regular deputies, same uniforms, same everything," Arpaio said. "So if you want to call me a publicity hound, good."