The University of Oklahoma’s K20 Center has a wealth of resources for educators!
One is K20 Center Games, which has a number of fairly impressive learning games appropriate for high schoolers. Once you sign-up (for free), they’ll send you an email (mine took a few days, but that might because it’s holiday time). Then, you can easily create a virtual classroom and enroll students in it.
K20 Learn is another excellent feature on their site. It has a wealth of lesson plans for all major subjects from grades one-through-twelve. I took a quick look through some of them, and they looked pretty good.
There are a number of other resources on the site, but those are the two that stuck out to me.
I’m adding this info to: