Keep on at the school, make an appointment to see them, make another one, then another one and yer you guessed it another one if needed.
Once the kids know the school wont allow it then it should be better for all.
Beating the bullies about the head will make you feel better but it really anit gonna help.
Altho they can't stop it outside the gates so some kind of self defence lessons at the local center might be worth looking in to, they are great for the confidence and making proper friends.