Will the future club members want to keep this project going or do you consider it just a one-time thing with you and your committee?

The reason for that question is to see what kind of expense you will want to pour into it.

I think you can appeal to the students and their parents for the first donation of seeds or plants/plant cuttings. There are many plants that could grow this way without having to buy even the seeds from a store or even buying clearance plants to keep things cheap.

You also have to check what kind of sun or shade conditions those beds are in, to have an idea of what might go there.

You will also have to consider about soil. Flowers do not need as much attention as vegetables do, so growing in native soil with a few amendments would be fine, but if you choose to go with more costly soil mixes, then that will factor into your budget also.

It seems to me that most such projects are done with annual flowers bought in six packs and transplanted into the beds so there is at least some instant gratification.