I'd advise a dry-run adhead of time of your camera making, exposure and development methods, just to ensure the bugs are worked out and the kids have a funner time than waiting around to see if you can make it work in class. For paper negatives I rate grade 2 paper around ISO 2-3. This means subject matter have to be brightly lit by sunlight, and exposure times of 15-45 seconds, depending on F-ratio of camera.
To help you get a ballpark exposure, I'll mention my metering method that seems pretty accurate with paper negatives (which have little or no reciprocity failure to deal with): I set my handheld meter (Gossen Luna Pro F) to ISO 3, meter the scene, then refer to the f-stop scale for an f-number that's 1/10 of the camera's f-ratio. For instance, if it's an F/150 camera than refer to the closest f-number to f/15 on your meter; then take the recommended time and multiply by 10. This will be your exposure time. For instance, if the meter suggests 4 seconds, then the recommended exposure would be 40 seconds.
To determine your pinhole's diameter you don't have to get real fancy and scan the hole with a scanner; just use a millimeter scale and loupe. Backlight the pinhole, position the metal so the hole is adjacent to your millimeter scale and you can guesstimate to within ~1/4mm the diameter of the hole. Then measure the focal length of the camera with the mm scale. Divide the pinhole diameter into the focal length; this should get you an accurate enough focal ratio of the camera to get reasonable exposures.
~Joe