My bluetooth is locked by the school

Even though you have graduated, your device is still being managed by the school. It is likely that there is a profile set up at Settings > Privacy & Security > Profiles. Usually they cannot be deleted on the device and will need to removed by the school. In any case, your school will be able to help, as the point of have Mobile Device Management is to prevent the user from making those changes.

I suggest that you contact your school IT admin/supervisor and ask them.