In Karen Keller’s kindergarten classroom, boys can’t play with Legos. They can have their pick of Tinkertoys and marble tracks, but the colorful bricks are “girls only.†“I always tell the boys, ‘You’re going to have a turn’ — and I’m like, ‘Yeah, when hell freezes over’ in my head,†she said. “I tell them, ‘You’ll have a turn’ because I don’t want them to feel bad.†Although her approach might anger some parents, Keller is sticking to her guns: It’s all part of a plan to get girls building during “free choice,†the 40 minutes of unstructured play time...