Continuity mistake: In "Hell-O", as Santana and Brittney are talking about the "hottest guys in the school", Santana's glass is almost empty. Yet two shots later, when she takes a drink, her glass is full. Also, when she says "Would you mind waiting in the car?" her glass is full and you can obviously see a slice of lemon in it. Yet in the next shot, when she says "And leave your credit card", her glass is only about 1/3 full and the lemon slice is gone.
Continuity mistake: In "Gives You Hell", while Rachel is sliding down the middle in the chair you can see Puck on the left then on the right. He couldn't move that fast in a second.
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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