My first thought is that you're attempting to connect to a different network than everyone else.
This type of login screen is an authenticated user. If it's just a password protected Wifi, there wouldn't be a username field.
Does he have a network login that he uses when logging into school computers? It would be like johhny@highschool.com Depending on the setup, you might leave off a portion of that.
If I was the school admin, I'd test the login on a school owned computer to ensure the login itself works.
Yes, he is assigned an email account and pw there has been an issue with his email account i think that maybe one issue. I think the issue was how his last name spelled out my last name plus my husband's. I'll have to see about that they may need to reissue one.
It seems to be a common problem with those older macOS versions, also on regular routers that use Wifi protocols that are "too new" somehow. A shame the iPhone 8 doesn't have that setting...