Dead Popular by Sue Wallman
A new, chilling read from the award-winning author of Lying About Last Summer. Kate Jordan Ferreira knows that you don't become the most powerful girl at school by playing nice, and Kate's ruthless.But when other students start revealing long-held secrets anonymously, she realizes someone is playing a much more dangerous game - and they know too much about Kate's past. If she doesn't figure out who's behind it all soon, her final year at Pankhurst could be exactly that: her final year.
Sue Wallman is one of the UK's top thriller writers for teens. Her books include Lying About Last Summer, See How They Lie and Your Turn to Die.