1) Practice is what you do at home on your time. You don't need anything other than your bass and your fingers.
2) Rehearsal is what you do in the jazz room with the rest of the combo.
Sounds like you're trying to practice in the main band room while others are playing. I can't see how that would work.
No, I think I get what's going on. It sounds like the teacher is trying to give the students independent work time. This is really good because it allows the kids to progress at their own pace. Strong students aren't held back by the slow progress of others, and it's easier for the teacher to give more attention to the weaker students who need it.
The problem seems to be that the school has a poorly-designed music room without any adjoining practice rooms, so everybody has to play in the same room.