Pagamo lets you – for free – create virtual classrooms for math and English (primarily vocabulary development).
It’s not quite as intuitive for teachers to set things up as other sites on The Best Sites That Students Can Use Independently And Let Teachers Check On Progress list. However, they have many short videos showing you how to use different features.
One of those features that makes Pagamo a bit different from similar sites is that it apparently allows you to “gamify” your assignments. I didn’t have time to explore that aspect, but it does sound intriguing.