They pay so much for such a crappy experiment.
For a real ecosystem and balance of nature lesson they should get a large 55 gallon and set it up with all live plants, wood, and different schools of fish of all activity levels and where they swim in the tank as well as shrimp, snails, and other algae eaters.
Then measure water levels daily, and make notes about behavior of fish in their environment and do a report on how all this benefits one another.
At the end of this they can find new homes for everything but keep the tank, stand, lights, wood, etc. That way they don't have to keep spending. Or just keep everything the same for every year to observe...