Notable is that the article is referring to a pre-delta study of elementary aged school children (elementary school normally caps out at around age 11 in the US). Young kids have long been known to be less prone to infection, less prone to suffering ill effects from infection, and less prone to spread the disease. Delta doesn't really change much of that, they are not the primary victims nor spreaders.

It's the older kids (who could have received the vaccine but opted out) who will have more "adult-like" patterns of transmission and should be more of a concern (not for their own health, really, but they might infect their elderly or infirmed unvaccinated relatives). One would expect masking to help prevent spread from these people.

But of course... these kids and their families likely already self-select and associate closesly with other anti-vaxers outside of school to begin with, so school seems like the least of their worries. At a certain point you kind of have to ask whether it's worth trying to protect people from themselves.