I just want to throw it out there, that this "dating" can also be VERY harmless. I know that when I was in grade 7 I had "boyfriends" but nothing ever happened. Holding hands, or a kiss on the check was seen as a big deal. And I'm sure that there are other kids out there who maybe might go further, but i really think they're the exception rather than the rule. This sort of heterosexual pairings are an important part of development, and i think that if you strictly place a rule on who they can and cannot hang out with, it might come back to bite you, or it may keep them from developing alongside their peers.

I'm by no means saying that you're wrong, but just that I think a lot of this dating is pretty harmless, and a natural part of adolescent development.