To unblock command prompt with Group Policy you need to do what lordoftheplat said,
Create a shortcut to gpedit.msc
Then open it and a window should pop up.
Double click on User Configuration
Then on Administrative Templates
Then on System
Below the folders that say User Progiles, Scripts ect... There are other things. To the right it says 'disabled' 'enabled' or 'Not configured'
You want to look for 'Prevent access to the command prompt'
Click it and go to properties and change the settings to 'disabled'
Click Accept and OK.
Now you should have the command prompt at school.
You can also unblock task manager and regedit this way.
NOTE -- This does not work on Windows XP HOME