All that weird stuff that happens is cause by a Boogeyman type deal. Everyone that it was the boy's overactive imagination but it turned out to not be. I think the monster ends up taking the boy, and the girl and some other guy go to the monster's world under the boy's bed.
I want to say the movie Scream but it isn't or maybe it's a Candyman movie? Because there're like two or three of those... What year did you watch it? Do you know?