Well if you are not even getting the prompt for the login, then there is an issue with the machine connecting to the network properly. As you should get it. I know cause i get it when i try to logon to secure websites that require a special input of username and passwords before they can even be accessed. Plus i connect to my secure XP Laptop from Win7 all the time as well.
So you need to check the network settings making sure they are setup exactly the same in Win7 as they are on the XP machines. Maybe contact the IT Support, unless you are it? If you are it, contact Microsoft. We dont know the credentials of the school so you might have security in place that is not normal for a home network.