How Did miss minchin react when she discovered the girls partying in the attic?

Isn't this in earlier chapters?

When the Miss Minchin caught the girls partying in the attic she was pale with anger and ruined their party . She punished Becky , Sara and Ermengarde for this and told them ," Lavinia was telling the truth ", so they knew that Lavinia betrayed them, guessed their secret snd told it to Miss Minchin .

In Frances Hodgson Burnett's book The Little Princess, Miss Minchin is furious when she learns that the girls are having a party in the attic. She noticed that her food and water supplies were running out more quickly than usual, so she suspected something was already going on.

When she ran into the attic and saw the girls playing games and enjoying the food they had prepared for dinner, she was furious. She scolded them severely, accused Sarah (the protagonist) of leading the whole thing, and she threatened to punish them severely.

Miss Minchin's anger was aggravated not only by the fact that her girls did not obey her, but also by her jealousy and resentment towards Sarah, whom she regarded as her spoiled and arrogant child.

Miss Minchin was determined to put Sarah in her place and make her pay for her defiance, but in the end Sarah's kindness and fortitude won out.