Apps stuck on log in loop

Hi all. I hope someone can help.

I work in a primary school with banks of iPads. Some of the iPads won't log in to certain websites accessed through Safari. The usernames and passwords are correct but rather than logging in they just go back to the log in screen with the username and password fields cleared. It isn't a fault with the website as other users are able to log in. Does anyone have any ideas or has anyone experienced this?

Many thanks everyone.