PHILADELPHIA — As America enters a less acute phase of the Covid-19 pandemic, it is time to reflect upon what we lost and what we learned. America’s failure to prioritize time in school should be at the top of the list.

Major disruptions to school schedules were perhaps to be expected in the early days of the pandemic. But we allowed them to persist to a troubling degree, even though we know that time in school is not fungible — learning lost now cannot simply be made up later...