How do I get in a classroom? How do I get assignments?
Classrooms are part of "Duolingo for Schools" (https://schools.duolingo.com/) and it is meant to be used by real teachers with their real life students. It is also used by parents who wish to monitor their children's learning and progress.
Sometimes a normal user creates a classroom, thinking it is going to be like a fun club or practice group, and they post the code or link and invite other users to join. Moderators delete these posts for two reasons:
The code/link exposes the "teacher's" email address, which is a security risk and against Duolingo's guidelines
The "students" who join the classroom give the "teacher" the ability to:
a. remove portions of the course
b. prevent them from using the discussion forums
c. see their email address
d. reset their password
Duolingo for Schools is a great tool in the right settings, but it's not something the average person would benefit from.