I agree with Rob.
- Set your sights on a goal, and work for it.
- Maybe get involved in a sport or club. Fill your time with activities that will help round out your college applications (I remember you saying you want to be a psychologist...) and keep you from having time to get in trouble.
- Spend time with students who are doing well, and avoid the ones you may have been running with when you got into trouble. This sounds tough, but if you want to stay out of trouble, you have to create a new environment for yourself. It doesn't mean you're "better" than them. If just means you want to try something else.
- Develop a relationship with a teacher/counselor who will be able to encourage you on a regular basis and who you can go to when you're having a tough day... and someone who will look out for you.
- Learn to control your anger, and don't let it be the other way around.
Good luck!