A century ago, only about a fifth of 15- to 18-year-olds in the United States attended high school, and less than a tenth of 18-year-olds graduated. Simply put, getting a high school diploma wasn’t viewed as necessary back then for most people to get a job.

But enrollment began to grow shortly after that period as more high schools were built and governments placed more emphasis on encouraging children to attend school and get their diplomas.