“Just” doesn’t mean what you seem to think it does. Schools have very little margin for soaking up absence at that scale. If one out of five teachers are out, the kids will not be doing the learning which is the stated justification for not keeping them safe at home. Teaching is hard work, not hitting play on a video, and it requires time to prepare in addition to classroom time – teachers don’t call in sick for fun, it actually creates more work for them since they still need to create the lessons and deal with the impact on their work when they’re back, and in many districts a decline in test scores will cost them money (in some, so does taking a sick day).

School administrators are extremely busy with all of the extra work they’ve been given, and if they’re babysitting students that work (often required by law) will not happen.