I always thought klasse was the room, as in classroom... Am I wrong? It means class, as in lecture?
"En klasse" kan be either a classroom or a class (as in a group of students that are always together for (almost) all subjects.)
If you want to tell someone that you're in class (referring to the lesson), you would say "Jeg er til time."
Why cannot I just write the number (30) instead of writing the whole word (tredive)?