The supplies can be a little expensive depending on how willing you are to rig stuff up yourself. You could probably go without heaters if the room all the aquariums are kept in remains at a constant, livable temperature. As for lighting, rigging up a shop light for t8 bulbs might be enough for all the aquariums, and I'd ask the school janitors to see if there are extra bulbs in the storage space. There were always some in the school I worked at. Small sponge filters for each of the aquariums would probably be the cheapest and most efficient, just add on a sizable air pump and some air tubing / splitters to power them all. Otherwise, a master test kit and water dechlorinator cover the base of everything else. Maybe a net and water siphon for water changes too.

If you want a cheap base substrate, I'd recommend either pool filter sand or black diamond blasting sand. You can get either from ACE Hardware, Lowes, Home Depot, etc. for very cheap. For the nutrient content theme you want, you can use organic potting soil or maybe root tabs to simulate it. As for plants, easier ones include java fern, java moss, frogbit, anubias, hornwort, water wisteria, dwarf sagittaria, and jungle val. Most of those will do fine in a wide range of temperatures and low lighting. You can also do some in each tank to show how different types of plants (root, stem, rhizome, and floating) are affect by the substrate nutrients. Fertilizer can be a bit difficult to go into detail with, but you could always do a comparison between using it and not using it. You could stock the tanks with some smaller fish such as guppies, but I'd keep it to only a few in each tank, considering that the tanks are pretty small and that the fish will probably reproduce. Otherwise, you could always do snails and shrimp because they'll do fine in small tanks and are helpful in the ecosystem.

For lessons, I'd start with tank cycling and the chemistry of it for the first month or so. Use the test kit every few days or weekly to track the tanks' progress. Maybe do a comparison between tanks with different scenarios (no plants + no fish food / plants + no fish food / plants + fish food / dosing starter bacteria). After all the tanks are cycled, you can experiment with lighting, plants, and algae. Some lessons can include how lighting will affect growth of algae and how the amount of plants will affect it as well. With guppies or neocaridinia shrimp, you can teach about reproduction and even offspring genetics. There's really a lot of stuff that goes into aquarium keeping that you can teach the kids in a nice, simplified way.