My father went to one of the last of the one-room schools, in eastern Washington state. He said about a quarter of the class would be listening to lessons at any time, the rest doing assignments or, if they had finished, a project chosen from the Book of Knowledge, an encyclopedia geared for that, that all such schools had; or, helping younger kids with their work.

The faster younger kids sat in lessons with the slower older ones. They all overheard lessons taught to older kids before they got there themselves.

Separating kids by age is historically new, harmful, and weird. Traditionally, younger learned from older, who learned better by needing to teach. The experience of teaching might be our greatest loss.

Right, because students were historically separated into groups by ... age?

...and zip code, which is very correlated with parental income, occupation, and education level