Schools in New York City fully reopen after 18 months of pandemic restrictions.

New York City’s classrooms reopened on Monday to roughly a million children, most of whom were returning for the first time since the United States’ largest school system closed in March 2020.

While the city reopened schools last fall for part-time learning, the vast majority of students chose to keep learning remotely. But with no remote option now available to almost all parents, classrooms will be full for the first time in a year and a half.

For months, Mayor Bill de Blasio has forecast the first day of school to be...