Google Classroom first showed up on the Play Store in 2015, as an organization tool for teachers and students. The YouTube for Android TV app is a bit older, as it first showed up in 2014, just one day after the Nexus Player went on sale. Now both apps have passed 10 million installs on the Play Store.
YouTube for Android TV's Play Store listing
Google Classroom's Play Store listing
Classroom currently has a 4.2-star average rating on Google Play, but YouTube for Android TV has a much lower average rating of 2.3 stars. Most of those low ratings came from the recent switch from a native application to a web wrapper, which resulted in some features being missing (at least temporarily).
You can grab both apps from the links below.