Welcome to Tropical Fish Keeping!
LasColinasCichlids brings up a very good point. It is not recomended that you plan the water around the fish (like with pH buffers etc), but that you plan the fish around your water.
Do you have a pH test kit? I recomend API liquid test kits. You can get a pH test kit for under $7 at Walmart, and it would be worth it to have fish that are happy, healthy, and at their peak coloration.
If testing will not be possible, prehaps we could come up with a (somewhat small) list of fish that will do fine from a pH of 6.2-8.
We would love to help you out with the classroom fish tank, or any fish tanks you personally have/want to have. If you have any questions, feel free to ask them here!