How many 'Brody Girls' were there?

I've read the book, seen the movie and the television adaptations, but it's been a LONG time. What I'm wondering is whether there were "Brodie Girls" for each year, in which case there would have been quite a number of them. The scene in the movie where she's describing Mary McGregor's death in Spain, there definitely were more than four or six girls gathered around. I suppose it could depend on how the system was set up... did she get an entirely new room full of girls each year, or did she keep the same girls during their entire time in the "Junior School," which I'm guessing could have been for four or five years? In the movie at least she continues to have contact with her girls even after they have moved on to the Senior School.