I don't want to jump to conclusions. Maybe something is wrong with the books. I think that I might head down their later, when I know the building is empty, and investigate further.
Check out the condition of the books first, before/if you decide to make contact.
(But even if they are in poor condition, I'm wondering if they can be pulped/recycled? Better than being treated as trash, especially if there are quite a lot of them, or this happens on a regular basis.)
I don't want to cause problems for the school and I love the principal who I know would be upset by this -- she taught in a low income school for many years so she is very aware of the hardship many children face.
You never know, boomerang, the principal might not even be aware
that all of those books ended up in the dumpster. It happens ... some department or the library has a big clean-up & that's the quickest way to get rid of unwanted old stock ...
I agree with you not wanting to embarrass the principal or the school, or make the book dumping some local "issue".
Sometimes the best resolutions can found through quieter, more diplomatic means.