My son only acts out at school: Advice?

I am at an ABSOLUTE loss with my ten-year-old son. He is the most amazing child, so kind, so smart. I never have to tell him twice to do anything at home; he’s so respectful to his father and me. Anywhere we go, he is amazing. Except for school. He has been put on a behavioral contract. He has three more write-ups, and he is being kicked out and put in a school for bad kids. He is constantly talking back to his teachers, telling them no, balling up his work, and throwing it on the ground. Just last week I had to go pick him up from a random parking lot where he was with the asst principal, a police officer, and the guidance counselor because the bus driver refused to go any farther with him on the bus because he was so beyond disrespectful to her and the other kids. Calling them “stupid” and “crackheads,” We do not speak like that around him. He was kicked off the bus for the remainder of the week. We have taken everything away from him—his tv, Xbox, phone. We came up with a rewards system to earn back time for the items. Nothing is working. He is still getting in trouble. His school calls me every day to either tell me to pick him up or to let me know how much trouble he is getting into at school that day. He spends more time with the principal than he does in his classes. We do not spank. We’ve tried having him write sentences, cleaning the house, redirecting. I absolutely praise him when he is good at school, but for every good day, we have six bad days to follow. We try to get him out more. Going on walks and playing basketball. He doesn’t have a lot of friends. He is kind of a loner. He is in intense Therapy and anger management. His anger is absolutely out of control. Anytime I confront him about school and getting in trouble, he becomes absolutely enraged. He was punching my walls, punching himself, kicking his dresser, yelling, and screaming at the top of his lungs. His school is great. They are really trying to work with him, but there’s only so much they can do. I’m just at a loss and so tired that I’m getting to where I’m giving up I don’t know what else to do. If you took the time to read all of this, thank you. Any advice would be amazing.