Prom Queen - S2-E20
Continuity mistake: As Kurt and Dave are talking in front of Kurt's French class, a short girl with a brown ponytail enters the class twice.
Continuity mistake: In the "Singing In the Rain" dance, the second time the three guys are on their knees, they splash water with their hands. The third time they splash, the water drops are moving toward the stage instead of away. And the rain in the background is falling up.
Continuity mistake: While Santana is singing "Back to Black", Jessie is seen obviously scribbling something down. Yet at the end, his notepad is shown with a drawing of a cat upon it. If he was drawing the cat, he would have been drawing smooth lines, not scribbles, as the drawing contains no scribbles at all.
Answer: The show is purposefully stylized and over the top. It is not designed to be an accurate, realistic representation of American high school. The idea that the glee club is so unpopular other students can literally throw garbage at them with impunity is an over-exaggeration of how glee club is considered "un-cool" compared to things like football. However, there is a second way of looking at it. The audience is simply never shown anyone getting into any trouble for throwing slushies. Just because we don't see it doesn't mean it never happened.
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