I feel like running away, just from reading about all this roughhousing.... But I wanted to share a little story with you that my mother used to tell me. She said that her older brother was quite a bully, and there was no adult supervision.
But when he broke her arm by putting all his weight on part of it at the edge of a sandbox, and she got a cast, she got the upper hand (until the cast came off)....

By the way, I would NOT encourage the use of self defense in school. School is more civilized than the home situation you've described.