Most anything that doesn't require cool growing temperatures would be fine to put in now. Veggies that are ready for harvest before fall would open up room for later plantings of quick growing veggies that would ripen in the fall.

Tomatoes, peppers, and eggplants could be planted now and should still be going strong in the fall.

Decorative corn is a fall favorite in our school garden and should go in soon. The dried ears can be left on the stalks for a long time allowing many groups to view the crop.

Lettuce, beets, and radishes could be planted now, harvested midsummer and replanted for another harvest in the fall.

Plant some frost resistant veggies later in the summer so you have some plants that will last after the tomatoes, peppers, eggplants and other warm weather crops freeze. Most of them do not require a long time to grow so wait to plant or plan on replanting later. The frost resistant veggies could include assorted lettuces, radishes, carrots, beets, cabbage, broccoli, and peas. A local gardener could suggest the best time to plant for a fall harvest.

We keep one bed for a "Life Cycle" garden and make successive plantings. We do not harvest all of the crop when it is ready but let it go to seed so students can see how seeds are formed for growing the next crop. Radishes, lettuce, and cilantro(coriander) can be planted every two or three weeks to give a good display. Make the first planting now.

Have fun with your garden. I've found raised beds are ideal when using gardens for education. A group can gather around the raised bed and everyone can see. No one has trouble knowing where they can step, either. Plus care of the grden is easier in my opinion.