Sometimes it pays to play detective.

A California high school student who hid in a gym locker to catch a thief ended up filming a teacher stealing cash from kids’ backpacks.

Justine Betti staged the undercover sting at Linden High School to bust the light-fingered lawbreaker who’d been making off with money stashed in book bags all year.

The sophomore initially hid in the locker, as classmates left the room, to see the gym teacher going through the bags.

But nobody believed her story.

So, determined not to let the crook off the hook, she staked out the room again on Feb. 13 – and this time was armed with a cellphone camera.

ABC News

Linden High School, where Justine Betti is a sophomore.

She also set up another camera in a second locker.

Sure enough, the footage shows the teacher – who has not been named by the media but is said to have been at the school for 30 years – rifling through their belongings.

“I didn’t want to believe that she would do something like that because she was so nice, but then she did it,” Betti told KXTV in Sacramento. “It was really scary.”

Betti took the incriminating footage to the principal, who promised to investigate and told her to delete it. But she had already passed the clip to her dad and friends.

ABCNews.com reports that the teacher has been placed on administrative leave pending an inquiry by the Linden School District and the San Joaquin County Sheriff’s Department.