Kathy, you do realise it is a British film?

Seriously I'm glad to see such interest from across the pond. Actually maybe one of you nice American types can clear something up for me that is referenced in a round about fashion in the film. In just about every U.S. film or TV show set in a school or college classroom, they show the class leaving as soon as the bell sounds. Even if the teacher is mid sentence, they just get up and go. The teacher doesn't bat an eyelid and just yells over them to mention homework or the next class.
Is that how it is in American schools? I find it odd because it would never happen when I was at school here. If I just stood up and walked out I would have been liable to find a board rubber bouncing off the back of my head! The teacher dictated when you left, not the bell. I'm not saying that the average teacher was an ogre who wouldn't let you have a break, but just that they were allowed to at least finish speaking!
The reason I mention it here, is that in Notes on a Scandal, there is a brief moment when Barbara (Judi Dench) is teaching, the bell sounds and the class immediately stands despite her being in mid-sentence. She glares and they all pause until she motions to allow them to leave.
I thought it worth talking about because it is a small cultural difference which you may have taken for granted. Perhaps even misled you about Barbara, but her actions are simply old habits and don't suggest anything other than annoyance at a breakdown in standards. Maybe this is becoming the norm now in British classrooms.