Fairness in our school administration?

recently,my son was suspended for 3 days.He was defending himself from another boy who refused to stop hitting him on the back.My son warned him,after taking it for a while,that if he did not stop,he wouldnt like it.The boy laughed and hit him again on the back,hard!My son pushed the boy backwards,knocking him down.Both are in 6th grade.The teacher was right there watching and did nothing until my son got up to defend himself,and then she sent them both to the office,where they were both suspended.I have a problem with that....! That the teacher did nothing and let the situation escalate, is unacceptable....the end result,with my son being suspended,was not fair.That he had to defend himself was her fault......She should have ended the tiff immediately and separated the two boys....but for some reason,she just sat there and decided to watch. I am thinking of filing a complaint against the teacher..Id like other parents input please....thank you very much.