If you have to have them setup for experiments, then maybe doing 2 planted tanks with exactly the same layout, but one using Co2 ferts etc, and one that doesn't to show the differences...
The shrimp tank idea is also a good one, since Cherry Red Shrimp now a days are pretty cheap and breed in any type of tank, Crystal Reds can be a little pricey depending on quality and where you are located, but the science behind getting them to breed might be a fun experiment, since you can test how different water hardness etc, effect how quickly or easily they breed...
So all in all, seems like there are a lot of options out there to choose from, depending on what the conditions of setting up the tank are as set by the teacher giving you the reins on this project...