Sometimes DuoLingo will only let a few students log in to a classroom at a time.
When my classes come to Spanish and try to log in, sometimes they can all login at the same time and other times it will only let a couple or three log in. When that happens, it won't even let ME log in with my user name and password and I'm the one that set everything up for the classrooms. I have 19 students that don't have anything to do in my class because it would only let 3 students log in today. After those other three logged out then I was able to log in and two others. Thanks.
Sorry to hear that you're experiencing this.
Could you please write email@example.com and include a few examples of student usernames? I'd like to look into this further so that it may be resolved.