the only reason you would need to prefix the log in acct would be if you are logging into a different domain than the one the machine is in. This also only works if there is a trust relationship set between the 2 domains.
I would take your machines, pull them out of the domain, delete the computer accounts out of AD, sysprep the machines to reset the SID and add it back into the domain.
From your post, you sound a lot like a teacher. You might want to call your district IT department. For assistance. You will need the unattended.xml file your staff created for windows 7 so you can run sysprep properly.
I am an IT guy in a 18000 user, 8000 PC, 9 high school school district.