yeah you would defiantly have to move the oscar to a larger tank when full grown but with the 18 " depth on the 40 breeder you could keep him there for a few years Id think as long as you keep up with your water changes.
I also have a 40 breeder my stock is
7 white skirt tetras
7 danios
1 gold gourami
2 dwarf butterfly cichlids
3 cories
1 common pleco (going into a other tank soon)
So its not like a biotope tank, just a collection of peaceful community fish. I think some gouramis or angles would be cool or the amazon tank i mentioned before. Some cichlids could be kribs, rams etc. or you could do some rainbowfish. I also like the idea someone else had of a tank with fish from your region. But do what you like and your students will get the most out of.