Perhaps she is not entirely wrong in that; the narrative states that all her girls are among the best students at the school. All except for Mary McGregor, who is unattractive, unintelligent, and blamed for everything, relentlessly if quietly bullied by her peers and mocked by Miss Brodie herself. Nevertheless, Brodie fascinates Mary as much as her five contemporaries: Sandy, famous for her small, piglike eyes, Rose, famous for sex, Eunice, famous for being athletic, Monica, famous for her anger and her mathematics, and Jenny, famous for wanting to be an actress. Brodie exerts a strong influence over all these girls from the moment they come into her class aged ten, which continues after they've left for the Senior School two years later. Indeed she is an interesting woman, all culture and no science - even making fun at one point of geometry and physics, because she doesn't understand a word of them - talking about Renaissance painters and history at times when she should have been teaching math. One of the ways she binds her students to her is by talking about herself, telling them about her fiancé, dead in WWI, inviting them to tea at her home, taking them to the theatre, and finally by confiding in them about the distrust and dislike the headmistress and the other teachers have for Brodie and her methods, thus making the girls into her trusted co-conspiritors.
In that way Miss Brodie reminds me rather of Julian Morrow, of The Secret History fame. Brodie's ideas about Goodness, Truth and Beauty mirror Julian's talk about grace and perfection, but where Julian is content to induce fascination in his students, Miss Brodie takes a further step and uses that fascination to manipulate the girls. She is autocratic and self-centered; she treats Mary McGregor with casual cruelty; she 'renounces' the man she loves because he is already married, but later tries to manipulate one of the girls into beginning an affair with him in a truly awful case of living vicariously through others.
Sandy is the one who sees her most clearly for what she is: egocentric, manipulative, immoral. In the last chapter she repeatedly points out that Miss Brodie is ridiculous and not to be taken seriously; in fact, Sandy follows her own advice and doesn't take her seriously for quite some time. It's not until the final shocking relevation that Sandy... snaps, in a sense. She has been growing more and more distant from Miss Brodie's teachings as she grows up. One catalyst for disobediance was the realisation that Miss Brodie truly, honestly, completely meant for Rose to have an affair with the married man whom she, Miss Brodie, loves. The final straw is the throwaway observation Brodie makes that it was she who sent Joyce Emily, a hanger-on of the Brodie set, to her death in the Spanish Civil War by encouraging her to run away and fight there: not for the Republicans, whom Joyce Emily once supported, but for Franco.
Part of Sandy's distancing from Miss Brodie is also the changing of her religious convictions; Sandy goes from regretting she was not brought up in a Calvinist household so she could have something to properly rebel against to becoming more and more interested in Roman Catholicism, which Miss Brodie decries as a religion for people who can't think for themselves. The all-knowing narrator, meanwhile, states that Miss Brodie would have been far happier there, that the Catholic Church would have somehow curbed her excesses. Not personally being over-fond of said Church, that was the one aspect of the novel that didn't quite ring true to me. Brodie is a mess of contradictions, socially progressive, politically a fascist, personally controlling while encouraging the girls to reject conventional authority figures (ie the headmistress). I don't quite see how making her Catholic would have changed the egocentrism which is her defining characteristic - and her downfall.
For Sandy, ultimately, betrays her, seeing no obligation of loyalty towards a woman who would imprint young girls with her own beliefs and encourage them to die for a cause not truly their own in order to feel some kind of personal satisfaction in a job well done: another girl rescued from the clutches of mediocrity!
And we're left with the memory of that awful statement of poor bullied Mary McGregor's: the years of Miss Brodie's class, she thinks, were the happiest of her life.