I've a very vague memory of a film I watched on TV about 12-13 years ago.
Made and set in the 70's I believe. It takes place in a medium sized American town and is about how the town's youths suddenly turn strange, become unexplainably nasty and sinister.
I can't recall what else happens, I think the kids maybe try to take over the town. It was quite spooky, although not all loud and action packed - more subtle and disturbing.
Possibly "Children of the Corn", which was based on a Stephen King short story (from "Night Shift") and has spawned six sequels. http://www.imdb.com/title/tt0087050/
It isn't chidren of the corn. Atmosphere wise it was similar to Carpenter's Assault on precinct 13, but without gunplay.
Not loud or action packed, quite subtle and unsettling. The spooky kids didn't seem to say much. They just went nuts.
I doubt I'd know the title if I heard it, but I've seen Children of the corn a few times . . .
How about this one? I've not seen it, and it's Spanish (but may have been set in America). I remember it being reviewed as "Would You Kill a Child?". http://www.imdb.com/title/tt0075462/ the comment has quite a detailed synopsis.
The Elijah Wood film Mattk mentioned is called The Faculty but its only a few years old. I think it was supposed to be a modern reworking of Invasion of the Bodysnatchers.