What happens to US education if there’s no one to teach?

Is “blar” a word? Karina Sandoval wonders aloud, chuckling as she marks up first grade papers at her Granby Elementary School desk.

It’s a mid-January afternoon at the rural Colorado school bordered by snowbanks, and it’s time to collect students from music class. Mrs. Sandoval settles them for snack time.

As natural as she is, she is not a first grade teacher. She’s a local parent who became certified to serve as a substitute this school year.

“It was an opportunity to help,” says Mrs. Sandoval, who has a kindergartner...