Teens usually have to sneak off somewhere "without adult supervision" to do the things you mentioned. This makes them an easier target. Its not so much what they are doing but where they are doing it.
Except for Freddy. He could get you right in the middle of a well lit classroom full of students...

Yeah but I always get the feeling that the people who make these movies feel like they deserve to get slaughtered for doing it hence why the goodie two shoes virgin is always the one to survive in the end, until she returns in a sequel of course.

Why do the survivors always die in a sequel? ::confused::

i think you're forgetting the obvious, Luv. The reason the many teens having sex get killed is because the film makers know most of the ticket buyers want to see both. The film needs to show the naked ladies (check), and then show a gory kills (check). Simple as that. That's why it's made that way.

The film producers are selling those two things. There they both are. The film wasn't made to suggest to people teen sex leads to being killed by a supernatural masked killer.

Then to end the film, the last person kills the bad guy. Audiences want to like and respect the hero, so they make them the good guy/gal, which traditionally has included a pure gal, but she was also a 'good person'; and if you can throw in that last chuck of audience that wants the hero gal not 'sleeping around', then bam, you got that too.