The Stunning Spell is supposed to knock the target unconscious. It didn't work completely, not because Dumbledore has any special immunity the way Luna does, but because Harry isn't very good at magic.
In the room on the top left, the students are – from left to right – Zabini, Crabbe, Millicent Bulstrode, Charles Warrington, Daphne Greengrass, Pansy Parkinson, and Graham Montague. We've seen Daphne Greengrass and Graham Montague before – they were the two students bullying Harry in McGonagall's class on page 8. And of course we've seen Zabini and Crabbe before.Approximate readability: 11.33 (454 characters, 90 words, 5 sentences, 5.04 characters per word, 18.00 words per sentence)