My child was misbehaving on the bus and got suspended,but this was after he has been bullyed all year?

My son is 10 years old, 55 lbs, and has not grown into his teeth yet. He is teased everyday, it has got so bad that he would run off the bus crying and wanting me to home school him. I have gone to the bus driver and told her they are hitting him in the head and saying they are going to punch his teeth out and calling him names. Yesterday I got a call saying he was suspended for hitting people and being out of his seat. My guess is he decided to take it in his own hands. This not like him, but if he did do this he should be suspended. I told the principal the things the other boys had been doing to him, he said he has no record of it and it didn't matter b/c sticks and stones may break my bones but words can never hurt me. I toke my son and left b/c I was not happy and started to rise my voice what should I do?