To be honest I would still recommend you wipe the machine fully and start again. Chances are that when you re-install Windows and you re-install all applications the school use it should be more than all your data put together, so will most likely be overwritten. This by any means isn't a guarantee, but is a high probability.
I have a pc that was use for school office and want to recycle it for reuse in the school. I don't want to completely erase/format the machine.... just remove any way or trace confidential files from being recovered. Is there any suggestion for preparing the PC back for use without the complete erase.