Other mistake: When Finn and Rachel sing Smile in the Choir Room in Mattress (Season 1, Episode 12), Finn doesn't appear to look at any other page but the first one in the music given to him by Rachel. It is unlikely that the entire score for the song was on the first page.
Other mistake: In the scene at the lockers with Brittany, Santana mentions having trouble finding a younger man per Sue's orders. However, Santana is a senior in season 3, which would make her a sophomore in season 1. Therefore, she could easily just date a freshman instead of needing Finn.
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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