Prodigy Game is a math site recommended by some teachers on Twitter (see their tweets below).
There are a lot of premium features, but teachers can create virtual classrooms for free.
Here are the tweets I mentioned, as well as a video about the site. I’m adding it to The Best MATH Sites That Students Can Use Independently And Let Teachers Check On Progress.
— Jim Cordery (@jcordery) November 11, 2016
— Liz Blose (@eduBlose) November 11, 2016