About 10 years ago the company I worked for (pharma) tasked me with closing an R&D facility that had been in operation since the 60s. Despite assurances that the chemical inventory logs were complete and up to date, you wouldn’t believe what we found in some forgotten closets and even offices. No joke, we found several small cylinders of phosgene (commonly used as a feedstock). It was insane. So I don’t find it hard to believe that a high school would have some nasty stuff in their stores. Hell, I had a chemistry set in the early 80s that would be a superfund site in a box today.
However, I will say I was surprised at how affordable removal was. We used a commercial company and it was much less than I would have guessed. Tens of thousands, not hundreds. A d we had a lot of stuff.