I watched it in 2002 or 2003 and it was about this boy who I think was bet by his friend to sleep with this girl who has a boyfriend and he finally manages to do so on a school trip seducing her whilst she is on the phone to her boyfriend. The boys then decide the girl was not enough of a challenge and so they set out to sleep with this other girl. The girl ends up making a plan with one of the boys to play the other by sleeping with him and then later telling the police she was raped because the boy said he had never been played whilst all the others had been.

Cruel Intentions 3 (http://www.imdb.com/title/tt0391891/)