I don’t think 2skep is doubting the odds. It sounds like they doubt that the high school is the >only< common link between the cases.

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.)

The article specifically calls out that amongst the people who reached out to him, "are former CHS teachers and staff members who didn’t live in Colonia, they just worked in the 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.

I understand where you are going with this, however, if they all drank from the same water supply, wouldn't parents also be susceptible? Anything else would be the same thing where the parents would also have been subjected to the whatever. The school is the one place where the kids went and spent significant time that the parents did not.

It's mentioned that a number of people didn't live in the area, they just worked at the school. That might be why the school is interesting.

Odds are not that low. In the 55 years since opening this high school would have had at least 20,000 unique people associated with it.

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.