“In some cases, the charter schools are taking them, in some cases privates and parochials,” said Glenn Koocher, who heads the Massachusetts Association of School Committees. “The bigger tragedy is that some kids aren’t getting anything, because they’ve fallen off the map.”

Mr. Koocher said he believes a third of the students that left public schools this year are in that category. “The districts have lost touch with them,” he said. “They’re staying home, probably doing nothing, and we’re out of touch with them.”

A reverse phenomenon has taken place at private schools, many of which began the school year with in-person learning. In New England, 36 percent of independent schools reported a rise in enrollment in September compared with last year, according to the National Association of Independent Schools.