Onyx is the biggest. They run an older version of Android, but do have the Play store. No idea about Google Classroom stuff. Keep in mind that other than as a reader these devices likely don't perform all that well. They don't run anything like state of the art hardware and aren't simply a modern Android tablet (like a Galaxy or ZenPad with a different type of screen).
Onyx is currently available mainly in Europe, but they do now have a US distributor who has stores in Brooklyn & Long Island NY (
https://onyxboox.com/buy). They say online ordering is coming...
https://onyxboox.com/news/44266
We have an Onyx Boox section of the forums here where a person can probably get more informed answers from folks who actually have one...
https://www.mobileread.com/forums/fo...play.php?f=220
There are a few other brands, but I know even less about them.