Teach and take are best.
"I'm taking a German class." "I'm in a German class."
If you're talking about a class in session, you are attending it, or in class. You're not watching it.
"I teach German." Or if it's already understood that you're a teacher, you might say something like, "I have three German classes right now."
If you are currently teaching a class in session, you are teaching, and not giving.