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.