I didn't fight with girls. I beat up boys who bothered girls, and I left the girls alone. I was on my own personal one girl playground patrol. I prefer kicking to pulling hair, even as an adult, though I haven't really kicked many people as an adult. Er, at least no one that I regret having stomped on.
So when my son gets in trouble at daycare, yet again, for fighting, I remember that I grew out of it, in my twenties, and fervently pray that this be the same in his case, or sooner.
Cornflower, you are a righteous girl. Shyness, vengeance, studies, and all. You should be proud of who you are.
Ask him why he's doing it.
Did somebody hit him and he's hitting back in self-defense to get him to stop? (this is fine)
Did somebody sit in his chair? (asking the kid to get up is a better than hitting. if he doesn't get up, ask a teacher.)
Did somebody just didn't do what he asked and your son is hitting because somebody disobeyed his command? (i.e. there was a disagreement and he resorted to initiating violence instead of 'agreeing to disagree and leaving each other alone'.)
Being in school makes all of this harder because the teachers don't explain anything, they just give commands. And when a fight happens, they punish both the initiator and the self-defender.
But anyway, the child won't "grow out of it". He'll either learn that it's wrong, and a better behavior instead, or he won't and will continue doing it. So it's important to help him learn why it's wrong, and a better behavior instead.
(if you want to discuss privately, PM me and we'll exchange emails)