I'd just read "and Jo" and Joey is irritating through most of it. Certainly doesn't act her age, or her position as head girl. I quite like the book as a whole, but Joey really needs Jem to be nasty to her a few times. 
It starts with her being told off for carrying Robin around, which she's been told not to-her response is "I don't do it so often since you and Jem made a fuss". Clearly she goes into teenage sulk after that, and Madge reflects that holiday Jo had "been difficult and tiresome, resenting any attempts at control on the part of her sister..."
I also wonder why on earth they tell Joey that the specialist is coming the next week, when it is obvious she's going to go demented that week waiting. Why didn't they either mention to her during the holidays and give her some time to recover-she obviously has no idea, because when she's asked how the Robin is at the start of term she says "much better, nearly herself again". Observation (also never noticing Madge was pg until baby arrived!) was not her strong point.
Or tell her afterwards. It seems cruel to tell her that week.
Surely if Madge feels she will resent not having been told the week before, she'd resent just as much not being told when they first sent for the specialist.
Then, knowing she was in a state all week, they know all is well, but don't phone up to tell her, until Miss A phones to ask.
Then actually Madge is quite rude to Miss A, saying all is well, but I can't tell you anything until we've told Jo... but you can tell Jo that's all okay, but nothing more. Why doesn't she say, can you get Jo to phone me back?
And the really nasty scene when Jo invites Frieda and Marie to her room and tells them not to tell Simone because she'll be upset. Yes, she would be upset, as, I think, any one of them would be to find that had happened.
There is also when Joey is first told about Robin's potential illness, she promptly says it was all Stacie's fault, and resists any suggestion it isn't. And then is quite nasty to her. Then when she finds out Robin is okay, Stacie says how thankful she is because she felt to blame, and Joey says "of course it wasn't, you mustn't think that" and Stacie "always felt a huge gratitude to Jo for saying it".
But the thing is, that it is clear that if Robin had been ill that she would have blamed Stacie.
Then the scene with Biddy appearing. I like most of that scene. What I don't like is at the end. The staff have told the group off, then the prefects summon them to be told off again. Surely that was none of their business if the staff have dealt with it? It's made very clear in many of the books that the staff feel that if the prefects have dealt with it, then they shouldn't, surely that respect is due the other way round. Would end up with very resentful middles that way.
And the silly idea that the extra food the middles are sneaking for Biddy being noticed. I hardly notice the difference in amount ate if I have one extra kid joining my 3. One extra child, even very hungry from a group of middles, say about 7 of them, would not be noticed.
Oops.