Asked whether that was an appropriate age to be protesting about a disturbing event, the principal, Danjile Henderson, said: “My fifth-grade students were very aware of the details of the events and wanted to have their own peaceful protest.”

Still, she drew lines around who could participate and how. Third through fifth graders may walk out; second graders can observe the protest, but not walk; kindergartners and first-grade students will remain in their classrooms for discussions on school safety in general that avoid the shooting itself. “Not all parents may want that detailed conversation on what happened in Florida,” Ms. Henderson said.

With some parents wanting their children to get firsthand exposure to a nationwide political demonstration; others worried that the protests are stoking the fears of young children about a threat that remains uncommon; and still others objecting to the gun-control message entirely, one question has been weighing heavily on school administrators this past week: How young is too young for children to join the walkout?