Continuity mistake: When singing in the choir room with Finn, Rachel's necklace changes from inside her dress to outside several times.
Continuity mistake: When the Warblers fill the cup up with rock ice, you can see the cup is empty, yet in the next shot it's full of Slushie and no rock ice is in there when it's thrown in Blaine's face.
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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