On a spring night in 2022, Ross Hill was trying to get several of his children—he has eight—tucked into bed for the night. *** “They were in tears because they didn’t want to go to school the next day,” said Mr. Hill, a 38-year-old teacher from Florence, South Carolina (pop: 39,958). Whether he knew it at the time or not, Mr. Hill’s family was part of a trend in the United States. A surprising number of children are miserable at school, research shows, and it’s a trend that began before the pandemic. For example, a 2020 Yale study that surveyed...