I only skimmed a few comments, and it seems no one is addressing another glaring concern you might need to address... Who is the boy? What kind of access does she have to him? How much time is she spending with him? How is she spending her free time outside of school and with whom?
All very important facts you need to know. It sounds like she already knows the mechanics of sex, and probably has gotten a healthy dose of the education mainstream media offers through t.v., music, etc., but it is more likely that this new side of her is coming from peer influence.
Is this boy inspiring these thoughts because he has told her this is what HE wants? Is there a friend who is encouraging these thoughts and behaviors? This stuff/talk/thoughts do not happen in a vacuum. She's coming in contact with a person or thing that is giving her these ideas. You need to find the source and nip it now.
You also need to make your expectations of how she conducts herself outside of the house clear now. And set clear boundaries...or as someone else mentioned...you could be a grandma not too long from now.
It's a different world out there. More and more I hear stories of trusting parents letting their kids (even as young as 8) have "innocent" sleepovers at the kid down the street's house only to find out that kid's parents allowed questionable behaviors to take place under their watch or worse...aren't even home.
Facebook, Google, and the such are a no brainer when it comes to looking for a source or outlet for your child to experiement or get access to people and things you may not approve of.
It's just time to get tough and do an inventory of everything. It's not being a controlling parent, it's being a proactive one.