YA novel. A teenage girl goes to live with her father after her mother has a mental breakdown. She falls for a rich boy who drives a red sports car. His previous girlfriend is missing. Turns out a memeber of the group pushed her over a waterfall and she died. They are all covering it up.
The last scene of the book takes place in the girl’s bedroom. She had not been to school because she was ill. The guy tells her how his ex-girlfriend was killed by being pushed over the waterfall. I believe it was first in a series. It was out of print when I read it years ago. Ideally, I would really like to find all the books in the series.