Regan called the school office before cycling after the thief for about 10 minutes, until the other boy got off the stolen bike to go through a gate.
Regan then confronted the teen, asking him to give the bicycle back.
After first claiming it was his own bike, the thief told him he could have it, Regan said.
The Year 12 student said he wasn't afraid of the other teenager.
"If he was going to do anything, I thought he would have done it already.
"I just thought 'I don't really want this kid's bike to get stolen.'"
The school and Rotorua police praised Regan for his actions.
"It was a very good thing for him to have done," John Paul College principal Patrick Walsh said.
"I can't do anything except commend him," said Rotorua crime prevention manager Inspector Stuart Nightingale.
"I loved the fact that he saw something wrong, and followed the offender on his bike.
"You should be able to do that quite safely.
"I'm really happy to see a member of the community step up."
Regan was only about 15 minutes late for his orthodontist's appointment, he said.
He said he did not want to chase criminals professionally - he was looking at a career as an electrician and planned to do an apprenticeship after leaving school.
Police were still looking for the offender.
