The League of Scientists offers a number of science-related games whose difficulty level can be adjusted. In addition, you can start your own games and have up-to-eight people compete in “your” race. It’s not as simple as it is in other game sites to create games that only include your classmates, but if you set it up that the people you want to compete against all sign-in at the same time, it seems to me that you should be able to make that happen. Also, unlike in some of the sites, you have to log-in before you play (registration is simple, though).
Even with some of those drawbacks, I’m still adding The League of Scientists to The Best Online Games Students Can Play In Private Virtual “Rooms”.