If you can't get the passwords from the school, I think you have to use an install CD.
What system is the laptop running? If it's an older system 10.3 or 10.4 (Panther & Tiger), there are cheap retail discs available on the web. But beware the grey discs being sold. These are the ones that came in the box with somebody's new computer and will only work on that model. You need the black retail discs to be sure things will work.
Alternatively, if the laptop is new enough, you may be able to install the latest Leopard system (10.5).