Re: So much for copyright!
If they were available in the school's library they were.
Now obviously textbooks are a rigged game, that's why there's a new edition every year (with basically the same content but the quizzes are different) so used textbooks lose value.
If you don't want someone using your website as "training data" then don't make it openly available on the internet. Require people have a login (even if it is free) before they can access it. Just don't be upset when Google can't index it and your search traffic drops to zero.