Say What? by Margaret Peterson Haddix

[S&S Books for Young Readers] Wait, what did you say? Sukie is worried -- her parents are acting strange. When she runs in the house, her dad asks, "If all your friends jumped off a bridge, would you jump off a bridge too?" When she eats peas with her fingers, Mom yells, "You'll put an eye out with that thing!" What is going on? She and her older brothers decide to investigate. And what they discover leads to a kids-against-parents war!