Being a man, I might have something to say about that.  The first thing that I would have to ask, is... is the girl his girlfriend, or a friend, was the touching invited or wanted.  Or is the boy just being punished by some teacher that caught him making out with his girlfriend?

Boys are boys, and boys like girls.  But if the girl did not want the touching, then I would have a serious talk with him about his actions and let him know how creepy girls will think he is, ruining any chance with girls at his school.  That is not ok.  Girls never really seem to like creepy guys.  Tell him he doesn't want to have that bad reputation following him around for the rest of his school days... which it will.

On the other hand, if this action is ok with the girl,( chances are, the actions have been going on for some time, and this time they just happen to get caught) there is nothing you can do.  All you can do is tell him not to do that in school.   If you tell him he cant touch, or make out with his girlfriend... only trouble awaits you.  Coach him on what you think is appropriate actions for boys his own age.