A majority of the nation’s 55 million school-age children have returned to classrooms, and that is cause for cautious celebration. The start of school in many places across the country was far from smooth, and there clearly are issues that need to be better addressed. But last year — which saw schools shuttered by the pandemic, face-to-face instruction replaced with improvised online education, kids cut off from friends and school supports — was calamitous.

“Work past the fear. Help your kids move forward. … Kids need to be back in school for their mental health, their physical health, their ability to develop socially — for so many reasons” was the message from New York Mayor Bill de Blasio as the country’s largest school district on Monday morning welcomed roughly 1 million children who had been sidelined since the pandemic closed schools in March 2020.

Complicating the school openings has been the spread of the highly aggressive delta variant. Pediatric hospitalizations have risen sharply since July, and while childhood covid-19 deaths and severe illness are still rare, the numbers are increasing. So it is understandable that some parents are jittery. What, they wonder, will the next few months bring? That a vaccine for younger children has yet to be approved adds to the worry. School districts already have had to scramble as they reassess and rethink policies and protocols that range from where students can eat lunch, to who should be tested, to what happens to students who are quarantined because they may have been exposed to the virus.

The Post’s education reporters examined problems that resulted from quarantines of students in Washington-area school districts. Just weeks into the school year, parents struggled to find child care, schools searched for substitute teachers and students were sidelined from class, a particularly maddening failure since districts had a full year of virtual school learning to figure out how to provide affected children an education from home. Complaints prompted Montgomery County to suddenly change course with a new regime of rapid testing aimed at cutting down the number of children sent home for symptoms that do not turn out to be covid-19.

Some educators and health-care experts said that guidelines for school operations, devised by the Centers for Disease Control and Prevention before the delta variant became dominant, need to be updated. Even so, far too many school districts have failed to follow the sensible advice — require vaccinations for teachers and staff, regular testing of unvaccinated people, universal masking, contact tracing — that has already been promulgated. It is no coincidence that districts that have imposed stringent rules, such as Los Angeles, have seen few outbreaks in the weeks since the start of school, while districts that didn’t require masks — some of which were forbidden to do so by orders of Republican lawmakers or governors — or other safety measures experienced mass student quarantines.

Hopefully, a vaccine for younger children will soon be approved that will give parents and educators yet another tool that will help keep students safe and in school.