If you take a group of 100 who attended the same high school they are also more likely to have drunk water from the same source, to have frequented the same entertainment venues, chilled in the same park and got their cars fixed at the same mechanic. Just because the all live nearby.
It is possible that they carefully investigated every one of these things and the high school is the only one which is common for all cases. But it is worth keeping an open mind. (At least asking what makes them so sure it is the high school.)
Which is why I suspect that they think its the high school.
Now I don't know if after 30 years they'll be able to find a lot of evidence of the source, but I'm no expert.
Expected number of cancer cases would probably be several thousand.
Claims of primary brain cancer derived from social media are probably not too accurate. A proper study is going to discard most as being confused with other cancers. So now you aren't looking at an unimaginable rate.