This is a common problem which pupils are having on their IPad. If you are repeatedly asked for Proxy authentication then try this:
Close Safari, double click home button and swipe Safari up to make sure it and any other applications are closed.
Launch the "Settings" app on your iPad through the home screen.
Scroll down the menu and select "Safari".
Scroll down to the bottom of right hand panel and tap on "Advanced ".
Tap on "Website Data" from the pop-up window.
Tap on “Remove All Website Data”.
Scroll down the menu and select "General".
Scroll down to the bottom of right hand panel and tap on "Reset ".
Tap on "Reset Network Settings" from the list of options (check that you choose this one!).
Enter the iPad lock code and it will shut down and restart.
Once the iPad has restarted login with your passcode, reconnect to the WiFi and start up Safari it will prompt for the proxy settings go to a website (BBC News Weather etc.). It may take multiple tries to apply the settings.