The National Football League has recently faced questions about its handling of concussions after four former players were found to have significant brain damage as early as their mid-30s. But teenagers are more susceptible to immediate harm from such injuries because, studies show, their brain tissue is less developed than adults’ and more easily damaged. High school players also typically receive less capable medical care, or none at all.

Interviews with players indicate that even those aware of the dangers of concussions ignore them. Coaches, trainers and parents can detect a gimpy knee or a separated shoulder, and act. But a concussion is often the player’s secret.