On a recent Thursday morning at Arundel High School south of Baltimore, Tiara Colbert divided her ninth graders into groups and asked them to write down all the stereotypes they could think of. “They can be awful,” she instructed, doling out markers to use on big sheets of paper tacked up around the room.

The kids milled around, jostling and joking with each other. And the sheets soon filled up: “Muslims bomb everything.” “All blacks are in gangs.” “Cheerleaders are stuck up.” “Asians are smart.”…