Margo, firstly well done for taking a project such as this on. I'm sure that you already don't have enough hours in the day.
I'm no expert, but to me the hardest part is going to be keeping interest and momentum going. I would imagine that one key element is going to be growing crops which will grow and mature quickly and in bountiful succession. Obvious ones being salad crops.
Herbs could also be very good. Not even so much because of culinery uses, but simply for the smells, both good or bad. Bad creating a laugh or a snigger. Gardening of any kind should incorpate fun. Crops could be planted in shapes and patterns, or even someones name. Depending upon how many students that you have, the beds could be divided up so that students could draw and design their own little patch. Even making labels and giving the plots names could all help to inspire.
Fruit could be a winner, if it doesn't get eaten first!
A summer picnic using the veg grown, could another good one.
Don't just stick to veg, grow a few sun flowers, the taller the better. Anything that inspires the imagination is good, and if the group as a whole could make up a story or two to go with the area, this could then be incorporated into their learning, especially if interest starts to lapse.