According to a later section of Goes To It, there are 10 girls from Monica's school who come to the CS:
‘How many are we?’ demanded Bill, as she helped herself to a cress sandwich.
‘Eighty-seven so far. Of course, we should never have got so many new girls this term if it hadn’t been for that Medbury school closing. As it is, we’ve got ten of theirs. I understand that Miss Cundell never took more than twenty-five at most, and when the school closed she had only nineteen. She told me that she had given the parents two terms’ notice, so one or two were sent to boarding-school at once. Three of the elder girls have gone to the grammar school at Weonister, which is seven miles from Medbury; the rest are, for the most part, either to go to other boarding-schools or have changed to a small day-school which had been begun in the same place. But they are all Juniors.’
‘And then there are the two Wallace girls from the Vicarage,’ added Bill musingly. ‘And that doctor’s girl from St Wynedd’s—what is her name?’
‘You mean Terry Prosser,’ laughed the Head.
We later find out that five of the new girls are in the Fifth Form: Jocelyn Redford, Clare Danvers, Myfanwy Tudor, Monica Marilliar and Ernestine Benedict. I can't remember how many of these are from Monica
Oooh - and I've just found another in the 6th Form:
Gwladys Evans, a former Braemar House girl
And there is also Iris Stephens, a new girl, in the 6th, but EBD doesn't specifically say whether she came from Braemar House.