well it all depends...is the guy bigger than you? if he is i say you get him from behind like he got her!
really though you got to realize you gota pick and choose where you choose to take care of it...the school wont recognize that you did it in defence of a girl ever...especially not after a day or more has passed, so expect to get suspended....that being said any guy who hits a girl deserves to be beat to a bloody pulp! even if you dont win the fight, if you land one solid one than you did all you could to straighten things out. most will tell you the longer you wait the more wrong it is for you to get involved...i say, the longer you wait the less he expects it!