Hi D.,
You mostly got great advice. Though to dispute one post I must say that these kinds of issues, including those mentioned for middle schoolers, happen in ALL areas in ALL schools. But you probably weren't disputing that, I just had to mention it!
Anyway, it sounds like you are very well aware of your values and position on the subject, you just need to refine it and define it for your kids.
We often get to thinking about what we don't want the kids to do, and forget to tell them what they can do. If your son is feeling some interest in the girls, you already know you can't talk him out of that. So have a conversation that sets up some parameters and helps him feel some ownership. If he wants to "go out" with a girl, what does that mean for him (and explain that you would like to know what it means for other kids, but that you have your own family rules and expectations for him). What CAN they do- sit together at lunch? swing together at recess? draw her a picture (not during school and one that you see first?), etc. This lets him help define going out, and gives you the chance to also say that you don't want him to kiss, hug, hold hands, whatever. Give him both sides. And because he'll be a part of the discussion, he feels some power over it. Which also means that you can say if his grades drop or other problems occur, you will need to revisit the issue and possibly not allow any of it. Maybe it will be an incentive?
Anyway, hope that helps a little. As a funny side note, my daughter is 9 and my husband told my daughter that she can not have any crushes, and further that no one can have a crush on her! Obviously my poor husband is having a hard time with the boys taking an interest!! But that's what got us talking about how to handle the situation at this age. He's settled down a bit, I can only imagine him when she is old enough to date. Yikes. I may have to sedate him. :-)
Good luck.