First, make sure a program or service isn't running that quietly prohibits using the Welcome screen.
The one that comes to mind immediately is a network service called "Client service for Netware."
Start>Settings>Control Panel>Network Connections.
Double-click a network device and look at the list of items associated with it. You must have "Client for Microsoft Networks" to do any simple local networking. If you see Client service for Netware, click on it once and then click the Uninstall button. It can be put back by clicking the Add button later someday.
When done, click Ok. If you didn't make any changes, click Cancel.
Now while you are in Control Panel, open "User Accounts"
Notice a link to "Change the way users log on or off." Click that link.
You should see 2 checkboxes. Check them both.
With "Use Fast User Switching" you can switch between students quite fast without waiting for a long startup procedure.
You must restart the machine for these changes to take effect.
If this did not fix your problem, come back, other things can be done.