Enough to know that kissing someone who doesn't want to be kissed is wrong, and rough-housing with someone who doesn't want to be rough-housed with is wrong. It's not complicated...
You're assuming she didn't want to be kissed (we don't have the girl's perspective on this; apparently they're "boyfriend and girlfriend", which I don't think are valid terms with six-year-olds anyway...).
Expelling imho is very extreme in this case, there's other ways to communicate to the boy/class that it wasn't appropriate without making a way-to-public statement about it.
