That doesn't mean transmission was rampant at school, just that lots of people who also go to school were infected. They also do lots of stuff.
Maybe they got it at school, maybe not.
Maybe a ton of people were notified directly about a contact, maybe not.
There's a ton of people in a school, there was a ton of community spread going on, of course there was a bunch of people in both groups.
That doesn't mean the transmission was at the school. Just that it was somewhere in the community.
We don't send out an email every time some at the school gets a splinter someplace during their day. If we did, you might think the school was a splinter causing machine with dozens of incidents everyday. Really, it would just tell you people get splinters and they also go to school. It wouldn't tell if you they got the splinters in school or not.
So, they might have and they might not, this letter isn't enough information on it's own. Now, if you got 80+ emails about a close contact, and with each of those one of the close contacts also became infected, then you could say that spread was happening at school since you're showing transmission between contacts.