Continuity mistake: When Mercedes is singing "Bust Your Windows", the blue rags on Kurt's car behind her change position several times. Also, Kurt is at first looking up at the hole in his windshield, then has his head down on the hood, and then is looking up at the hole again, over a series of 3 or 4 shots.
Continuity mistake: When Will reads the newspaper review of their concert, the date on the newspaper is February 12, 2009. This date is impossible. Most of the Glee kids who graduated in 2012 were sophomores in season 1, and the only way to be in school around the February 2009 date is if the class of 2012 were 9th graders. Only this is not the case. Also, all the football or basketball episodes in season 1 would all be incorrect. The most likely date for Acafellas is September 2009.
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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