Continuity mistake: During "Never Been Kissed", just after Sue activates the confetti cannons and is talking to Will, the amount of confetti on Will's shoulders changes between shots.
Continuity mistake: During season 2 "Never Been Kissed", just as the Warblers begin to sing "Teenage Dream", a boy with a briefcase walks in front of Kurt, heading towards the left. Two shots later, the same boy with the breifcase walks left in front of him again.
Continuity mistake: While the Warblers are singing "Teenage Dream", during the closeups of Kurt, his bag's strap is under his jacket's lapel, but in the long shots, it's on top of the lapel.
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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