Have you looked at the National Gardening Associations Kids Gardening web site for ideas?

Here is a link that might be useful: kidsgardening

Take a look at the Cornell Composting site. They have some school project ideas.

My idea would be a comparison of various methods and sizes. Use the same materials (grass, leaves, coffee grounds) to make a:

3x3x3 mesh bin
5x5x3 mesh bin
4' round bin 24" high (my favorite size)
bin in a trash can
Lasagna bed
Pit dug into ground

and compare the temperatures in the core (daily? weekly?) even during the winter. Show sample of each in a jar in the spring.

Here is a link that might be useful:

Here is a link that might be useful: City Sprouts

Check out our program at Granny's Garden School. We have been working with kids in our school gardens (100 vegetable beds, lots of flower gardens, nature trail and apple orchard) for 10 years. You do not say what is your goal with restarting the gardens. We believe in planting with a purpose in mind. The students plant potatoes in the spring to harvest in the fall. Certain flowers in the spring to press and/or collect seeds in the fall, etc. Go to the website below and click on School Garden Program and Garden Adventures for ideas.

Here is a link that might be useful: Granny's Garden School