It's a kind of old film (I think) and as far as I remember, there's one girl who kills the other members of the group she's with, one by one. There's a scene where she's in a small boat with a member of the group who is usually in a wheelchair, she takes life jacket off disabled girl and kisses her, then pushes her out of the boat
Would be grateful for some help with the film title

This is a wild guess:

The In Crowd (2000)
IMDb synopsis:
A mentally disturbed young woman takes a job at a posh country club and falls in with a clique of wealthy college kids where she's taken under the wing of the clique's twisted leader, who harbors some dark secrets too terrifying to tell.

The In Crowd (2000) - IMDb (http://www.imdb.com/title/tt0163676/)