Sybil/Josette's accident is one of those "happening off set" scenes that are very disappointing from what you hear about later.
Another one is Phil's polio, plus Margot's frailty/spoiling. It felt like there was a book missing about the former, and the latter is only mentioned it retrospect.
They're all things that sound really interesting to see what happened and how they're handled-and much more realistic senarios than the constant falling down the mountain side.
I feel very sorry for Sybil really. All the awe the children seem to have of Len should really have been hers by rights as the daughter of the founder. It would have been nice to hear a couple of new girls saying "You're mum is really Madame? Who started the school? Wow!" rather than always "really Josephine M Bettany?".
She is the second child, and regarded as not very bright from fairly early on. When David is obviously bright (medic later) so overshadowing her there, and she has Bride snapping on her heels academically too-aren't they same form in Lavender despite Bride being younger?
She's not regarded as behaviourly good, academically good, sporting, musical, popular, a leader... her only points are good at needlework and very pretty. She's even passed over for prefectship until the prefects ask that she joins them. Which, if the rest of the Bettany/Russell/Maynard crowd hadn't been so dominant in laurels, could look like being careful not to show favouritism-but it didn't matter for the rest of them. It isn't even as though there was someone who was terribly obvious for head girl that year. Can't even think who it was? Perhaps Loveday?
And her aunt, who everyone else thinks is wonderful, has a real down on her. Can't you imagine that having an effect on how the school/staff view her? 
Mopsy the chapel stuff is totally OTT. Can you really imagine the forms delighted that they can give their pocketmoney to build a chapel. They must have had a good amount of pocketmoney to be able to donate enough to make a difference, plus afford sale stuff without spending it on anything else. 
And it was a real anticlimax when I read it for me. Len squeals and says she has a germ of an idea for a real form contribution to the coming of age, but needs to talk to "mamma" to really work it out. Sounds intreging doesn't it? Must be something really complex.
"Start collecting to build our chapels" 
Well that really must have taken Joey's mind to be able to work it out to that level mustn't it? And how does that become such a form thing either. It's not really even a school pupil thing, it's the old girls and the ever devoted staff that donate mostly.
I think the staff should have been reminded of the bit in the Bible where Jesus says that you should give in secret and to your own conscience, for if you make a song and dance over it, you've had your reward. Was it Biddy that starts by saying she'll give 10% of her salary? Surely it would have been considered extremely bad taste to say that in front of everyone? Discussing money was considered bad taste at that time wasn't it?