I'm pretty sure the girl's name was Fern, and she becomes obsessed with her teacher. When he dies (I believe he drowns) she kills herself and haunts her best friend by tapping on her window. There's a fish (or at least I'm fairly certain it was a fish) necklace involved and the girls hide notes to each other in a tree hollow.
I would have read it in around 2000 and I don't think it was that old then, so I'm guessing the book was published in the mid to late 90's.
I'd love help finding this book! If anyone could even point me in the direction of the publisher.
I'm fairly sure there were a series of these short horror stories from the same publisher with different authors - I seem to recall one about a clique of vampires maybe at a high school.