You raised a kid who thought it would be acceptable to punch a teacher?
The school didn't "let" one of their (note spelling) teachers (note lack of apostrophe) "provoke your child into punching him." Your child should have known better -- as civilized human beings, we don't just go around punching people, no matter how bad we think they need it; and believe me, there are a few people reading this question who might think YOU need to be punched.
Just a tip: If you call an attorney's office about this situation, the person on the other end of the phone line may laugh at you.
Just another tip: If there are teachers reading this question, they may be laughing at you as well, laughing to keep from crying. "letting one of they're teacher's" ??? Oh come on...
Sit down WITH your child so the two of you can write that letter of apology TOGETHER, because seriously, at 17, she's old enough to know better. "We don't hit" is one of those simple things that most kids learn by kindergarten or so.
Common sense, which isn't near as common as was once believed.