On a sweltering July afternoon, Solomon Carson, 6, jumped off the stoop of his family’s tidy rowhouse in West Philadelphia, full of what his father, David, called “unspent energy.”

When a stranger asked his name, he answered brightly, but added that he couldn’t spell it. “I can help you with that,” his father said, patiently pronouncing each letter, with Solomon repeating after him.

Solomon was supposed to have learned the basics in kindergarten this past year, but his first year of formal education was anything but.

When Covid-19 closed classrooms,...