Continuity mistake: When Sue and her mother are singing "Ohio", during the closeups, Sue has her arm around her mother's waist, yet in the wide shots, Sue's arm is hanging by her side.
Audio problem: During "Marry You", in the wide shots where Carole is coming down the aisle and the Glee kids are in the background, the lyrics and their mouth movements don't match up.
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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