Honestly, go to every kids' house who knows the combination to the locker with one of your parents, then talk to each kids' parents and see if they've noticed a new bass in the house recently. Don't say you think any of the kids stole it. Instead, make it seem like you thought someone took it home for practice and left it home by mistake, and that you need to get it back to the school ASAP for school rehearsals. Ask them if they wouldn't mind looking around the house or have you help them look for it. Once you find it (if you do) take it to the music teacher/principal immediately with your parent and tell them who had it.
And yes, report it missing to the police.
And yes, report it missing to the police.

