Terrible, terrible idea.
I think I can sum up a logical course of action:
-Get off Talkbass. I know this is hard for you, but it can be done.
-Heart to heart with the rents, not mentioning the gear. Find out if there's more to the story, just not this story. It may change nothing, but at least it'll put you and your mom on better terms
-School help. Begin with the your friend and the teacher, then go either guidance and/or principal. A detention is a bad thing, you might as well see if you can get it removed. And your mom will respect no one more than the principal
-Then, after the school authority/authorities have had a discussion with the rents, go to your mom again, this time talking about the gear. If she still will not repent, she really is off the deep end.
-If she does not repent, you need assistance outside of the school. Counseling. Not a cop. Counseling.
-Don't get the law involved, that would do far more damage in the long run than the loss of a nice rig
-If nothing else works (and many are going to disagree with me here) I would move out. Shack up with a buddy, or the Y or something. Just for a while, give yourselves some separation.
-Remember: Teachers/school officials have a very difficult time admitting and correcting there mistakes, and would much rather just ignore the situation and hope it goes away. You will need to be annoying, and hope that your teacher is willing to help you out. If not, I fear all may be lost.
Keep us posted!