More than half of schools in England either restricted attendance or closed during teacher strikes on Wednesday, government data suggested.
Teachers in England and Wales were among thousands of workers taking action during what was said to be the biggest strike day for a decade.
University staff, train drivers, bus drivers and civil servants separately walked out.
Most were taking action over pay not keeping pace with inflation.
Department for Education estimates on school closures released on the day were based on attendance data from 77% (16,400) of state-funded primary and secondary schools in England.
