I don't think it makes any difference if the kids are ill enough to not be in school or not, although it's pretty unlikely that the majority of them are ill enough for 10 days isolation, and the associated health loss.
What matters is that it's not proving to be an effective intervention, it's not working, there's high levels of transmission. So it's pointless, either the high level of transmission is okay, in which case there's no need for the measures, or the high levels of transmission is not okay, so we need to bring in more measures that work.
The one thing that is pointless is measures which achieve nothing, they cause (a small amount of harm) and make it harder to put in place meaningful measures when they might be required.