Is mona A

because Mona was friends with Jenna Cavanaugh and saw the girls shoot fireworks into Toby's treehouse, which is when Jenna was blinded. So she never forgave them and got revenge when she found Ali's diary in the trash on the side of the road which had all of the girls secrets. therefor, Mona decided to follow thru and get back at them in honour of Jenna.