YA Love Triangle set in a prestigious New York school

[Sam]

This young 18 year old girl, gets accepted into this super prestigious school in New York. She gets there and falls for the head mistresses right hand man. All while her best friend, (who's name I'm pretty sure was Simon) pines for her. Anyway, about halfway through her time there she learns the headmistress is turning the students into Sucubi.

A minor detail, that may also help.
At some point she kisses her best friend but thinks it's the right hand of the head mistress, but later she kisses the right hand man and the kiss is totally different so she's all confused about it, but at the very end of Le book, she kisses Simon and figures out it was him.

[Kris]

Sam, around what year did you read this book?

About how old are the main characters?

[Lobstergirl, au gratin]

Sam, are you still looking for this or did you find it?