Yeah, I'd second that comment. There's a LOT of options out there for relatively dirt-cheap optical mice. You might want to just do a PriceGrabber or MySimon search, or even go to TigerDirect or wherever. And if you're running Panther on those computers, then the fact that all of these mice have two buttons will be a non-issue for support, since Mac OS X by default has always supported two buttons (and the scroll-wheel).
Just be mindful that if they're being used on certain kinds of surfaces, "optical" mice don't track very well. Such surfaces include wood grain and certain kinds of smooth or textured light-colored plastic surfaces. The easiest solution to this would be to get a cache of mouse pads. You might try DriveSavers.com or some of the other companies out there who give away these things as advertising promos and see if they'd consider donating these pads to your school as an advertising opportunity.