I Know What You Did Last Summer by Lois Duncan

Julie, Ray, Helen, and Barry make a pact to never tell anyone what happened when late one summer night, they accidentally ran over a little boy on a bike, killing him. A year later when Julie gets a mysterious note without any indication of who it came from with only the words "I Know What You Did Last Summer" on it, she starts to panic. And she's not the only one in the pact who's receiving mysterious notes and threats. Someone knows what they did that night and they want revenge for what the four are desperately trying to keep hidden.