After thinking about this some more, and being a parent myself now, I think I've changed my mind on some things here.
First of all, you're going to need to deal with teasing in your life. Maybe the boy likes her and doesn't know how to present that properly? They're 8 year olds, their actions don't always make sense to us. Hell, if I really liked a girl I'm sure I wouldn't present myself right either, NOW.
So is the answer that when you get teased, you punch people in the face?
Is it just okay because it was a boy that teased her, and she's a girl? Can she hit girls who tease her? Can boys hit girls that tease them now too, following the same logic?
Imagine if the table was turned, your girl teased a little boy, and he "punched her in the mouth". How would you feel about the boy? Would you be happy if he went home, was not grounded, and his parents could hardly hold their laughter in? You would go apeshit.
It's not okay to hit people at school because you got upset. It's only okay to hit them back, because then you're already in a fight.
Second, I think it's natural to be proud of your kids for standing up for themselves, however a lot (98%?) of that is the parents own ego coming into play. You like to think of your kids as strong, so checkmark there. You like to remember when you had kid fights, checkmark there too.. You kind of selfishly like that feeling of power, that your family was the "winner" of something. ....When I say "you" I don't mean literally, nothing personal, just in general.
Nobody wants to see their kid on the shit end of the stick in any situation, yet there are winners and losers in everything. I think most parents that complain to teachers, complain to principals, write letters and generally try to protect their kids at school are doing nobody a service but themselves, and are being very selfish.
When my son is old enough to go to school, I'm going to do my best to stay 100% away from how they do things there. I can't control it, it's not my decision, and I trust them to make responsible decisions. If they drive me too nuts, I will have to move schools. However I won't complain to teachers or the principals about anything. If my son has a bad day at school and everything went against him - well, I hope he learns some life lessons, which are going to be 100000x more valuable than whatever they taught him that day at school anyways.
You have to let them fall down, you have to let them get back up by themselves, and you have to give them freedom to make their own decisions, even at 8 years old. If something is not "fair" then welcome to life my boy.