Are you logging into a domain of some sort? what type of login screen do you have?(Welcome screen or Old style[basically if you have to click your user rather than type it in a "user" text box you have the welcome screen])
I'm unsure if the domain is changeable in the welcome screen but try clicking on the other things in the welcome screen to see if you can see a "Domain" text box and ensure it's the name of your pc..
If you can not log-in as administrator then you are pretty much out of luck, I suggest to contact the school IT department and they should be able to help you since they have all of the log-on information.
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