OK, you will still need one user account - call it student or something. That way, it can be limited by the administrator (to keep people from messing with stuff they shouldn't be).
By default when setting up the machine, if there is only 1 user account with no password, XP will boot directly to the desktop. I'm not sure if this will be true after the fact (removing accounts to get down to one user). If not, post the conditions and wether home or pro & someone will get you through it.