6-year-old boy suspended from school for kissing student on the cheek

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.