I think everyone is agreeing with you. Even 8th to 9th is quite a difference. Regardless it is still a sub making an inappropriate choice about what to show your children - if it's borderline, then it shouldn't be shown. Or at least, parents should be notified.
BBM is only borderline, IMHO, for two scenes - a very short, quickly cut sex scene and a murder/mutilation scene after the fact, not during. Those two scenes I would distract my 4 year old from (or turn off) - but I wouldn't shield her from the rest of the movie - there's no reason.
However, if I had a 12 year old, I would likely allow them to watch the entire thing, if I were present. I would, though, be very upset if they were shown it at school, due to those scenes and because I wouldn't be able to control the discussion after.