Well, that's the thing, isn't it? I don't think for a moment EBD was conscious that she was writing a scene in which the power of a charismatic, wildly popular sister-of-the-Founder HG over a gentle, ineffectual and unwilling Head is being demonstrated, but it's certainly open to that interpretation. Especially when it's followed shortly by the whole Matron Besly fiasco, which largely comes down to her not having been properly briefed on school rules (prefects don't get order marks, are allowed to read in bed, and staff don't enter their bedrooms) and not realising she's supposed to recognise that Joey is a 'dear girl' and be given special treatment.
This is one of the things I find most compelling about the CS, and why IMO it supports so much dissecting (and also fanfic for the same reason). There's this gloriously frequent disconnect between what the author definitely wants to say, ever so didactically, and all the other stuff which definitely happens and which is completely at odds with the deliberate message! It's like her own world is misbehaving. 
I love the dormitory descriptions. As a child I just straightforwardly enjoyed them (although I'm sure i had no clue what a 'cretonne' was). These days it's a sort of nostalgic affection for the absurd. I love the lengthy discussion of new dresses in one of the later books (Ruey?) in much the same way.
Also, Margot Venables. As a child I didn't really notice that there was anything odd about her just 'fading away', but isn't she like... 40?