Actually, Tiz, the school might very well have taken action against your son, I'm sorry to say. Most school have a NO violence policy and that includes self-defense (repulsively stupid, I know). I know this from experience. When I was in high school I did exactly what your son did and ended up getting a one day suspension for it, the only trouble I EVER had in school. The instigators got 3 days. The school understood the situation and was very sympathetic to myself and the friend I stood up for, but they said if we were involved in a fight they HAD to suspend us.
From someone who got bullied a lot, that list is actually very good. Staying in groups, although not a permanent solution, is helpful, as is ignoring bullies and removing yourself from the situation.
