400 Bad Request error

Sorry to hear that, I'm not sure what could be. A 400 error is typically an error on the client side that prevents the request. Is it possible that you're using an old custom link to a worksheet? If you could provide the links that you're students are using to access the site, or more details on exactly what they're doing and the urls or screenshots of what's happening, we're happy to help. You may also want to check to see that the local school firewall isn't preventing them from accessing the site now. Ruling these things out will help troubleshoot the problem.