I would say to charge a flat rate, either by the hour or per person, and then give the students an option for purchase. I know it seems like a pain, but I have done a couple sprots teams (I have a mom who won't let any team her kids are on be photographed by anyone else, she cracks me up) and a couple fundraisers. They are no brainers in the editing department and you'd be surprised how much money you could make for an afternoon of work. I only do maybe 2 or 3 things like this a year and it's nice cash. I think it would drive me nuts if I did more cause that's just not what I want to spend my time doing, but 1 or 2 times? No big deal.
Make up an envelope with A, B C, packages (very few choices) and charge the school whatever you decide. Who knows, you could easily make a couple thousand? Even if each student only paid you $15 for a 5x7 (or something like that) you'd still make $1800. Make a generic announcement for the school that you'd drop their photo into and include that as an option and charge $3 or $4 a card. Most trade schools don't provide students with a way to order graduation announcements. There's lots of possibilities. PM me if you want to brain storm