For fall planting, I agree that greens would be a good choice. Spinach is easy to grow, and can stand some cold weather. A lot of kids are pleasantly surprised at how much they like spinach raw as salad greens, or on a sandwich. Lettuce is another easy to grow crop, and many varieties like Black-seeded Simpson are called "cut and come again" because you can cut the leaves off 1/2 inch above the soil level, and they'll grow back.
I don't think anyone has mentioned garlic yet, but that is another good thing to plant in the fall and harvest early summer. Peas can take a good bit of cool weather, as can root crops like onions, carrots, and beets. In fact, I've read about people heavily mulching their carrots and harvesting throughout the winter, as it is supposed to make them sweeter after a frost.
Do you only want plants from seed, or would you be interested in something like strawberries? They take a bit to get established, but they'd come back every year, and you'd be able to pick them in the spring during the school year. Rhubarb and asparagus are other crops that take a couple of years to really get up to speed, but are easy to care for, and could be harvested in the spring. I'm trying to think in terms of what would be ready to eat during the school year, as so many crops are traditionally grown during the summer vacation months!
You might also want to look into something called Winter Sowing. This would be a great way to extend the gardening into the winter months, and would give you plants early enough to see some results before the school year is over. You can read more about it here:
http://garden.org/ideas/view/v...
www.wintersown.org
