Rather than paying a student for good grades, see if you can get this money for your cause — find sponsors who are happy to pay a certain amount for every A and B that appears on a report card. You could even have a bonus for someone who manages to get straight As.
As well as raising funds it's a great motivation for students to keep studying hard and to hit the best grades that they can, particularly if the good cause is something they have a particular interest in.
It's important to work together with the teachers on an event like this. They might be willing to keep running totals for you and could let the students all know where they currently stand. Extra tuition could be supplied for those who need it.
Let each of the students photocopy their report cards to send along to their sponsors, together with envelopes (if necessary) for collecting the money. Include thank you notes from the students explaining what the funds are going to be used for.
Celebrate the end of term or exams by throwing a pizza party to say thank you to the students for their hard work and the money they've raised.
