Professor McGonagall’s irritated voice cracked like a whip through the Transfiguration class on Thursday, and Harry and Ron both jumped and looked up.
It was the end of the lesson; they had finished their work; the guinea fowl they had been changing into guinea pigs had been shut away in a large cage on Professor McGonagall’s desk (Neville’s still had feathers); they had copied down their homework from the blackboard (“Describe, with examples, the ways in which Transforming Spells must be adapted when performing Cross Species Switches”). The bell was due to ring at any moment, and Harry and Ron, who had been having a sword fight with a couple of Fred and George’s fake wands at the back of the class, looked up, Ron holding a tin parrot and Harry, a rubber haddock.
“Now that Potter and Weasley have been kind enough to act their age,” said Professor McGonagall, with an angry look at the pair of them as the head of Harry’s haddock drooped and fell silently to the floor—Ron’s parrot’s beak had severed it moments before—“I have something to say to you all.
“The Yule Ball is approaching—a traditional part of the Triwizard Tournament and an opportunity for us to socialize with our foreign guests. Now, the ball will be open only to fourth years and above—although you may invite a younger student if you wish—”
Lavender Brown let out a shrill giggle. Parvati Patil nudged her hard in the ribs, her face working furiously as she too fought not to giggle. They both looked around at Harry, Professor McGonagall ignored them, which Harry thought was distinctly unfair, as she had just told off him and Ron.
“Dress robes will be worn,” Professor McGonagall continued, “and the ball will start at eight o’clock on Christmas Day, finishing at midnight in the Great Hall. Now then—” Professor McGonagall stared deliberately around the class.
“The Yule Ball is of course a chance for us all to—er—let our hair down,” she said, in a disapproving voice.
Lavender giggled harder than ever, with her hand pressed hard against her mouth to stifle the sound. Harry could see what was funny this time: Professor McGonagall, with her hair in a tight bun, looked as though she had never let her hair down in any sense.
“But that does NOT mean,” Professor McGonagall went on, “that we will be relaxing the standards of behavior we expect from Hogwarts students. I will be most seriously displeased if a Gryffindor student embarrasses the school in any way.”