The second part of chapter 15 might be thought of as the whiplash point. Lavinia, the head Mean Girl, has told tales on Ermengarde and sent Miss Minchin in an unprecedented second trip up to the attic to catch the girls in the midst of their pretend princess banquet. The extremity of MIss Minchin's anger can only really be understood as sparked by the disruption Sara brings to the proper order of things. Viewed from a distance, why should it matter that Ermengarde chose to share her food "care package" with Sara and Becky? Why should it matter that she shares her books? Why should it matter that Sara rearranges bits of rubbish that have been left in her room into a make-believe feast hall?
To be sure, there are Rules about students not leaving their bedrooms at night, no doubt. But that alone can't explain the magnitude of Miss Minchin's response, or why it is directed primarily at Sara and Becky. The only thing that explains it is the fracturing of class boundaries implicit in the fraternization (sororization?) and the evidence of Sara's continued defiance of her "proper place"--a place that doesn't include having dreams and fantasies. Peak outrage is generated by Sara's insistence on the equivalence of her position and Ermengarde's. When Miss Minchin scolds Ermengarde, "What would your papa say if he knew where you are tonight?" (i.e., in the attic with servant girls), Sara, in one of her quiet reproachful comebacks, asks Miss Minchin what her papa would say if he knew where she is tonight.
All are punished: Ermengarde confined to bed the next day and reported to her father, Sara sentenced to another day with no food, and Becky told (although falsely, as it turns out) that she will be thrown out in the street the next day. And Sara is left to contemplate the broken fragments of her "pretend" before falling into an exhausted (and still hungry) sleep, repeating (and foreshadowing) her fantasy about the attic containing a warm fire and comfortable bed and a hot, filling supper.
This is where the previous chapter comes into play, in which Ram Dass and the secretary discussed their plans. Because we can jump past the mechanics of how the attic is transformed and have Sara wake to the results of that transformation. It takes some time for Sara to assure herself of the reality of the gift, if only because it matches her fantasies so closely and so elaborately that it seems unbelievable that it could exist anywhere but in her own head.
I would quibble about the plot convenience of Mr. Carrisford identifying his gift anonymously as "from a friend" except that this is perhaps the most believable aspect of the whole non-communication of identities foundation of the plot. Carrisford wants to be the magical anonymous benefactor. It's more fun that way.
And--true to Sara's nature--once she has convinced herself of the concrete reality of the gift, her immediate response is to wake Becky and share it with her. And, as we shall see from the next visit of The Magic, Mr. Carrisford is belatedly shamed (though perhaps that's too strong a word) into explicitly including Becky in the bounty by providing her with her own dishes and transfering Sara's now unneeded bedding to Becky's room to supplement Becky's own. But this only emphasizes that The Magic isn't about simple charity, it's specifically about rewarding Sara for being a person worthy of charity. But we have gotten ahead of ourselves into the next chapter.