Continuity mistake: When Rachel and Finn are doing their toast, the amount of cider in the glasses changes between shots.
Continuity mistake: Just as Kurt is pulling up a chair next to Blaine in the Lima Bean scene, Blaine's arm is down by his side. Yet when Kurt hooks their arms together in the next shot, his arm is on the table.
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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