i believe you didn't list your equipments to invite criticisms on how well off your family is, but i guess that is the envy of some too. i'm not into event nor studio, but my personal opinion is that treat the work as a service and the photographic portion as your interest and learning. do not treat it as a business deal and do not allow your fellow school mates to be treated like clients. let them know you are helping them out of good will and shooting out of interest.
if the equipment is your own, and there is nothing that you need to buy, then essentially the shooting can be for free. what is being put in would be time and energy, assuming it is done in school compound and take no transport charges.
calculate carefully on the cost if there is any. perhaps add a 20% margin or more to guard against loss due to mistakes, and just be open and not worry, since the margin is to avoid a loss, and any profits is to go to school. what is important is you keep things as cheap as possible for your fellow school mates.
unless you really have interest in desktop publishing and making of stuff like photo frames, why dun you keep things simple by just providing the digital copies, in cheap cdrs? it makes things very much simplier, and reduce monetary handling. if you are helpful and resourceful, you can recommend on a list of places that do great printout, or help to collate orders to an external agent and provide collection on behalf on the couples and arrange a distribution date/time for them to collect in school? in that case, all you did is a service, and the cost of printing is decided commercially by the external agent, which will solve your worries of how to set the cost. expenses cover the cdrs and miscellaneous + 20% margin, the rest is up to the couple if they want to print on their own or to get you as a middleman to collate an external order where they pay for what the external agent has projected.
students tend to get very excited and creative. i'm pretty sure it is hard to convince y'alls not to spend anything on decorating the booth and to minimise on things like advertisement, brochures and banners. basically a lot of expenses may have been incurred out of too much enthusiasm that is not practical. i dunno but that is how i feel when i look back.
as for photography, it is hard to suggest. basically i think it is something like wedding photography in open compound. and that is best to leave to the photographer. you can perhaps arrange 1-2 assistant for the photographer, which i think will help. and dun worry about ideas, couples themselves probably have plenty of ideas. you can even encourage them to pre-think what they would like to shoot but this takes quite some communication skills and it depends on inter-personal situations.
for good or bad, it is for fun and interest, both for the photographers and the students. but if you take it as a deal, you will see problems arising from expectations.
manage the load well. how many people will take it up? how much time will you allocate to each couple? is there a limit you would want to set to prevent overloading? that is the 2nd major problem.
and last of all, perhaps important, perhaps not. one thing about the copyright. if engaged as a profit or deal, the copyrights goes to the couple, and if it is done without being a deal, the copyrights goes to the photographer. usually for most things like school events, nobody is going to care about all these things. but somehow i thought pictures for couples maybe a personal thing. if the pictures are meant for just the couples, i'm sure it is al'rite if you keep it as your personal portforlio, but do bear in mind if you ever need to publish the photos, e.t.c as part of an article in a school magazine talking about your ECA's events, unexpected problems of misunderstanding can arise if communication is not done properly.