Hmm, my high school had lockers where we had to bring our own locks. You picked your locker and could change it if you wished, though of course, it had to be to an unclaimed locker. But there were lots of empty lockers in my HS when I was there (30 years ago), though, because we were in a sort of enrollment dip (I was born in the early 60's, sort of between the end of the baby boom and the beginning of generation X). In HS, they told us we had to empty our lockers over Christmas and spring breaks so the Janitors could clean and fumigate them. If you left your lock on during a school break, they'd cut it off. Our gym lockers in HS, and our Jr. High lockers had built-in locks. I don't know if it was possible to change the combo on those. They likely would have just changed your locker assignment if there was a problem with someone else knowing the combo.
Though now that I think about it, maybe they did have a way of changing the locker combos? Seems like previous students might remember the combo of their old locker, and if they were jerks, they could break into their old locker and rob its current owner. They told us to never leave anything valuable (like money, jewelry, wallets etc) in our school lockers. Which was a bit hard with PE lockers when you dressed out in gym clothes that didn't have pockets for these things. Honestly, though, I don't think the teachers and administrators worried about things like that back when I was in jr. high and high school. Kids pretty much had to deal with everything--bullying, theft, harassment etc. on our own, and if you complained to the teachers or school authorities (or even parents) you were mostly likely told you brought your "victimhood" on yourself by provoking the bullies or thieves in some way.
So it may depend on when and where your story is set. If it was more than a few years back, and there aren't any free lockers to switch the kid too, the school officials might just shrug and say, "You shouldn't be leaving valuables in your locker anyway."