They masked 6+ year olds and did not mask anyone under 6 in school.
You'd expect higher rates of Covid in the 5 year olds than the 6 year olds.
In fact, it was the exact opposite - rate of spread was perfectly correlated with age. Masks showed no impact.
There's also interesting data from North Dakota (https://twitter.com/TracyBethHoeg/status/1470072066380419072).
This is a great dataset because it controls for a lot of variables - it's two school districts in the same county; one optional and one mandatory. The mandatory masking county consistently has had higher rates of Covid.