Had a similar issue, some 11 year old boys where picking on my 8 year old son, my other son who is 11 saw them doing it, beat 3 of them up. I got a call from the school, telling me what had happened and I had to come in. I go there sit and listen to them telling me that my older son should not have done anything and should have told the teacher, which he had done previously but the school done nothing and my 8 year old had to hide in a toilet for lunch. I went ape shit at the school refused to back them on telling my son not to get involved if it happened again.
Took my boys to local restaurant for dinner that night, very proud of him for sticking up for his brother (they hate each other and fight everyday)