A few points:
1) You're not giving us the whole story. Don't take that the way it sounds. I'm taking an Interview and Interrogation class for my Criminal Justice degree, and I can see that there are details that are missing. I don't want to get into what details, or why they are missing, because that involves me making assumptions, and that's a bad idea.
2) Did YOU buy the amps from GC? If it's in your name, and you can document the funds transfer from your account to hers for the purchase, SHE HAS NO LEGAL RIGHT TO DO WHAT SHE DID. While she may be your legal guardian with the rights and privliges that that entails, she does not own YOUR property. Therefore, she cannot do what she did. Go down to the GC with your bank statements. I assume that your GC operates like mine, where anything bought in from the street has to remain in a back room for 30 days in case it's claimed to be stolen, so they won't be sold to anyone else for a month.
3) Talk to a cop. Don't do this in an official capacity. Don't go down and open a case against her. Just tell him the story as you see fit, and ask him if you have the legal ground to get your amps back. While I'm pretty sure about them still being your property, because you are underage it might be hard for you to get them back. So talk to the cop. He might also have some suggestions for you. If he (or she) is on your side enough, you might be able to convince them to come talk to your mom. If they are willing to do that, make sure that they do so OFF DUTY. If they go on duty, it all gets logged, at which point it would be very likely that your mom would go to jail for theft. That would not help things at this point, only make them worse. This would ruin whatever vestages of a family you now have. But if the officer is off duty and shows up in an advisory role, instead of an enforcement role, it might help your mom see that what she did was wrong.
4) Talk to your school counselor. They are (supposedly) good people who are on your side. They might be able to sort out whatever difference of opinion you are having with your teacher.
Personally, I'm not a violent person. My family is fairly nice, and I was never abused or even hit by my mom or dad. Spanked, yes, but that's different. Having said that, if my mom did that to me, well, I'm not sure if I could keep from being violent towards her. EVERYONE who knows me knows that you ask before you touch my music gear. I'm a generous person with just about every aspect of my life, money, food, car rides, my time and effort, etc, but NOT with my music. If my mom and come home and told me what she did, I probably would have laid her out. I'm not saying that you should do that, I'm saying that just how angry I get thinking about it makes me realize how bad the situation would be.
Rock on
Eric