I have got, and read, my book - thanks so much! 
I really enjoyed it, it didn't go the way I expected at all...
Spoiler alert
I agree there are certainly aspects of sadism in the way the characters are treated. One could argue that with Mary MacGregor, there is something of the martyr in the way that she accepts her role as a scapegoat - she suffers uncomplainingly, nobly perhaps, with no resentment or malice in her character - but the constant contemptuous scorn with which she, and her death, is treated/described is utterly lacking in compassion. Mary is described as "stupid" countless times, "too stupid to lie", "habitual slow bewilderment", always "a lump", "lumpy" , lump-like", and that's by the omniscient narrator.
The way her death is described by the other girls is "tragic" but the way Sparks describes her death, and the way it is foreshadowed in the lab - "hither and thither she ran in a panic between the two benches" [between two flames] - it is as if Sparks thinks Mary was too stupid to live, and deserved her fate. It feels like victim-blaming, as if she deserved to be bullied. But if Miss Brodie hadn't been engineering/enforcing the dynamic, then Sandy certainly would have modified her behaviour to be more kindly towards Mary.
If you draw parallels between Miss Brodie's beloved fascists and the way she sees herself as a leader of girls, then taking on Mary as the scapegoat in order to draw the rest closer makes sense. But the way Mary is treated by the narrator/author - in particular her ultimate fate - sits uncomfortably when viewed with that lens. I would've expected a bit more sympathy/compassion directed toward her.