Cutting the janitor (I suppose we're supposed to call them custodians now) some slack, it's a perfectly natural thing to say under the circumstances.

It still sounded really stupid at the time.

I hear there are now safety features in lockers to prevent lock-ins like this. Either there weren't any like that when I was in elementary, or the lockers at my school were antiques. Maybe both. Anyway, I'm assuming here that this school seriously needs to upgrade their lockers. (Come to think of it, those lockers did look pretty old, so maybe they were originals. And that building was nearly 90-years -old!)