I've done stuff I've regretted out of anger, too. Listen, when you're so depressed and upset about school, it is a LOT harder to control the anger.
When you feel yourself starting to get mad, you need to work on consciously telling yourself to calm. In the case of the classroom incident, instead of losing your temper, perhaps you would be able to go to the bathroom, and take the long way there and concentrate on calming yourself down. Deep breaths. Tell yourself you CAN control yourself. Because you can. It is really, really hard, but you can do it if you force yourself to.
As for school, can you ask the teachers for extra help if you don't understand something? Or is there anyone else who might be able to help you make sure that you know the material?
Also, whether you get that calculator back or have to get another one, make sure to put your name on it in a permanent way. Then if someone take sit, you can tell the teacher it is yours and tell her it has your name on it. Then there won't be a need to throw anything.
Best of luck. You CAN do this. Don't let anybody tell you that you can't