Corrected entry: During the Herbology lesson with the mandrakes, Professor Sprout issues the class earmuffs, obviously to block out the screams that would follow. In order to do so, the earmuffs must block out all sound entirely, lest they all pass out, assuming the slightest sound from the mandrake causes ill effects. The plot hole is with the earmuffs. When the first mandrake is pulled, the rest of the class quickly press their earmuffs tightly over their ears. This action (and their faces) show they can hear the screams to an uncomfortable level. A moment later, Neville faints but more or less out of fright. Sprout states afterward that he's neglected his earmuffs, Seamus then replies that he's just fainted. Seamus heard what Sprout had said, given he had the earmuffs on and the distance between he and the teacher. They all should have fainted after Sprout pulled the first mandrake (this can also be said for after they all pulled up some 10 or so afterwards) as they could all hear the cry through their earmuffs.
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Correction: These are only juvenile mandrakes (in the book they suffer from acne and misbehave), and their screams are much less powerful than fully-grown plants, which the students probably would not be allowed to get near. (It is mentioned that these plants have to be mature before they can be used for the petrified antidote, and it's never stated that their screams must be completely blocked out, only that ear protection is needed.) Therefore, the students only need to wear the ear muffs to deaden the sound, not entirely eliminate it, allowing them to hear Professor Sprout speak. As you pointed out, Neville did not faint from the sound.
raywest ★