Teach your students to ensure there’s a delay between requests so they aren’t hammering anyone’s server, and follow the rules in the robots.txt. I’ve scraped more than a billion pages without any issues.
Actually many of the students are technically competent to do the scrpaing mostly using Python and I am pretty sure they learned not to overwhelm web servers.
I'd say the act of web scraping alone, is almost never unethical if you are careful to not cause undue load to servers. From an ethics, not legal perspective, I don't see a whole lot of difference between your computer's silicon eyes and your organic eyes just looking at something that's already in plain view.
It might be illegal in some jurisdiction; IANAL but I think you can just get out of that jurisdiction and scrape away if that is the case. It might violate some ToS but ToS isn't law; the consequences of violating a ToS are usually on the order of getting your IP banned.
What you do with the stuff you scraped can be ethical or unethical.