My son's school is discriminating against him! Can I sue?

At recess today, some ignorant kid called my 11 year old boy a fat loser, and he got mad and they started a little fight (no big deal though, they're in 2nd grade and kids fight sometimes). Then my son threw a brick at the kid to end the fight, BUT ONLY HE GETS IN TROUBLE!! The principle made my son write a letter of apology and personally deliver it to the other boy in the ICU. The other boy didn't get in trouble, but mine did? My son is traumatized from the whole experience. I think I should sue. What should I do?