JERICHO, N.Y. — Fifteen eighth graders at Jericho Middle School were considering a fictional case of stereotyping by hair color the other day, or how a boy came to be prejudiced against people with green hair, or “greenies.” From there, they extrapolated to the stereotypes in their own lives: dumb football players, Asian math whizzes, boring bankers. “We can feel stronger going back to our hallways,” the teacher, Elisa Weidenbaum Waters, said, “going back to our homes, going back to our society, and saying: ‘You know what? What you said is a stereotype, and that’s not cool.’ ” This year,...