Factual error: In season two the show makes Dalton look like it's in a suburb of Lima. Dalton Academy is suppose to be located in Westerville, OH, which is two hours southeast of Lima. Kurt & Blaine would not be able to go to the "Lima Bean" coffee shop every day if this was the case.
Continuity mistake: During "Blame It on the Alcohol", when Quinn is shouting at Puck, her hair is messy and straight. Yet a little while after, during "Don't You Want Me", her hair is curly and styled perfectly.
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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