A couple of caveats with using OHP film: Instead of basing the design on the Pi board's actual size, include an extra fraction of a millimetre to allow for the surface contraction on the inside of a fold when you bend the film though 90 degrees (and for human construction tolerances).
If your box base base turns out to be a quarter-millimetre bigger than necessary, it'll probably still look okay. Not quite so super-duper-impressive snug, but okay. A box base that comes out a quarter-millimetre too small after folding, OTOH, is likely to look a bit icky. There's no "squeeze tolerance" or "play" with OHP film, as there normally is with, say, corrugated cardboard or card
Also, decide whether you want your printing on the inside or outside (depending on whether your artwork in more likely to be scuffed by the board or by what's on your desk, and on whether you want a more matte or more glossy finish). If you're printing on the inside surface, remember to reverse the design!
I think a lot of schools will probaby have the facility to photocopy onto acetate, for OHP purposes ... in which case you could send them the template printed on paper (along with the board's promotional materials), and they could photocopy it onto OHP film themselves. Printing directly from PDF or a vector artformat would be better, but require more knowledge.
I think we might be coming back to the issue of needing a central file area for the R-Pi project, not just for images and 3D-model files, but for other file formats like PDF, Inkscape / etc.