Sasha looked at her own results. Her "diamond test"—pressing her back to the wall and seeing if her hand could slide through the gap at her waist—showed she was doing well. However, Denis realized he needed to stop slouching over his phone.

"Posture isn't just about looking tall," he told the class. "It’s about how your lungs breathe and how your bones grow."

Mr. Petrov stood by the whiteboard, where a diagram of a perfect human spine was sketched in green marker. "Today," he announced, "you aren't just students. You are orthopedic specialists."