The student-to-teacher ratio is one of the things parents consider when judging a school. If you’ve got too many children per adult, parents begin to worry that little Susie won’t get the attention she deserves or that unsupervised boys will form feral gangs that will turn the place into a scene from Lord of the Flies.

On a national level, it seems, that problem has been solved. The United States added millions of adults over the past decade, but we forgot to replenish our children.