wisdom needed : post here

It was Zack's Parent-Teachers meeting last night. Overall he's come back with great feedback, except for the hitting of classmates. His teacher has put it as "zero tolerance of physical aggression". Obviously he's not alone and there are a few boys that are setting one another off (I saw it happen in the playground yesterday when another little boy was playing with Zack after school, on the slide, and he just hit Zack as they were sliding down. Zack then retaliated).
Keith and I have been talking about this for the past few weeks, as it's been in the headlines. It is so much about the how of dealing with it, and what to do. Zack does thrive on positive reinforcement, so the carrot of praise is ever before us. The real challenge however is the initiate, the beginning, the 'what starts it'? question.
We have drummed it into him that he cannot hit other children, (just like we used to say, "no biting"). We have drummed it into him that if someone hits him, he must tell the teacher. Immediately.
I feel like he's getting mixed messages though. If Calvin hits him here at home, he tells me, and I say "Calvin is very naughty, isn't he?", and then I tell Calvin off. But there really isn't any justice in that is there?
Talking to my counselor, she's said that it's just massive emotional need that exists within children of this age, and they can't tell you what or why. So if Zack is full of emotion and someone hits him, going and telling the teacher seems like an insipid plea for justice in that situation. I really, really need to get my head around this.