To get out of ninth-grade science period one recent Friday, the King twins had an excuse that is so very 2021. Alexandra and Isabelle, 14, had to miss class — including a test — because they were participating in an actual science experiment: a clinical trial of Moderna’s Covid-19 vaccine to evaluate whether the shot is effective and safe in children ages 12 through 17. “In science we’re learning about, like, genetics and stuff like that,” said...