Obviously the health officials did not talk to anyone even loosely affiliated with an actual school. For reasons big or small, terrible or sympathetic, parents send children to school sick all the time.
Schools have spent decades encouraging nonsensical levels of attendance, rewarding kids who don’t take a single day off all term/semester/year, and even punishing kids who take “too much” time off sick. Kids who actually get ill enough to stay home miss out on fun things and “prizes” and awards, because going to school with the flu (or almost anything else) is considered right, good, and necessary.
Then a pandemic comes along that solidly half the population insists is “just the flu” and you think people are going to forget that you indoctrinated them to believe that if they were capable of being conscious (and sometimes not even then) they should be in school? (And later, in work – this is at least part of the point of this stuff, training you to work while sick).
To say nothing of the simple fact that parents are frequently without childcare options and are forced to be in work even if they should be isolating. That’s also a huge problem.