Continuity mistake: In 3x09, while Kurt and Blaine are singing "Let It Snow", the fire is going. When they finish the song and notice the "audience", it's out. When Kurt is opening his present from Rachel, it's going again.
Continuity mistake: In 3x09, during the opening number, Kurt is wearing the gum wrapper ring he received as a gift from Blaine. Although the scene where he receives it was cut from the aired episode, he wasn't supposed to receive the ring until later in the episode.
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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