In the Fifth at Malory Towers by Enid Blyton

Moira had to keep her hands well down to her side, she so badly wanted to strike her sister. Bridget kept well out of the way. She always came off worst in a struggle.
    There was a pause. “You’ll be sorry if you do report me about this afternoon,” said Bridget at last. “Very sorry. I warn you. Go and report Maureen! She’ll expect it of the domineering Moira! But just remember — I’ve warned you — you’ll be sorry if you report me .”

    “Well, I shall,” said Moira. “It’s my duty to. Fourth-formers aren’t allowed in these practice-rooms, you know that.”
    She turned and left the room, still trembling. She went to find Miss Williams, the fourth-form mistress. If she didn’t report those two straightaway, whilst she was furious, she might not do it when her anger had died down.
    Miss Williams was rather cool about the affair. She wrote down the two fourth-form names Moira gave her, and nodded. “Right. I’ll speak to them.”
    That was all. Moira wished she hadn’t said anything. She felt uncomfortable now about Bridget’s threats. How could Bridget make her sorry? Bridget was so very fierce sometimes, and did such unaccountable things — like the time when she had broken every single one of Moira’s dolls, years ago, because Moira had thrown one of Bridget’s toys out of the window.
    Yes, Moira felt decidedly uncomfortable as she walked back to the common room. Bridget would certainly get back at her if she could!

The plot is successful
    MAUREEN had been rather scared at Moira’s sudden arrival in the practice-room. She had heard the angry voices in the next room too, when Moira had left her, and had been even more scared. It didn’t take much to scare Maureen! She slipped hurriedly out of the room and went off to the classroom to put the finishing touches to her designs. She was to show them to the others that evening.
    She saw Gwen’s sour face as she walked into the common room with her sheaf of designs, and sheets and sheets of music and verses. Oh, Maureen had been very busy! If Mam’zelle and Miss James had known how hard she had been at work they would have been most surprised. Neither of them had any idea that Maureen had it in her to work at all.
    “ What they taught at Mazeley Manor I really do not know,” Miss James said to the other teachers each time she corrected Maureen’s work.
    “Self-admiration — self-esteem — self-pity,” murmured Miss Williams, who taught one lesson in the fifth form, and had had quite enough of Maureen.
    “But not self-control,” said Miss James. “What a school! It’s a good thing it’s shut down.”
    Everyone was in the common-room waiting for Maureen, though neither Gwen nor Catherine knew the little plot that was being hatched by the rest. Maureen beamed round. “Now you’re going to see something,” she said, gaily, and laughed her silly little laugh. “It was always said at Mazeley Manor that I was a good all-rounder — don’t think I’m boasting, will you — but honestly, though I say it myself, I can do most things!”
    Maureen was surprised to hear some of the girls laughing quite hilariously.
    “You’re such a joker , Maureen,” said Alicia, appreciatively. “Always being really humorous.”
    This was a new idea to Maureen. Nobody had ever called her humorous before. She at once went up in her own estimation.
    “Now,” she said, “I’ll show you the designs first. This is for Cinderella’s ball costume — I’ve gone back to the sixteenth century for it, as you see.”
    Shrieks of laughter came from everyone. “Priceless!” said Darrell, pretending to wipe her eyes. “How can you think of it, Maureen?”
    “A perfect scream,” said Mavis, holding up the crude drawing, with its poor colouring. “What a joke! I didn’t know you’d such a sense of humour, Maureen.”
    Maureen was puzzled. She hadn’t meant the drawing to be funny at all. She had thought it was beautiful. She hurried on to the next one —