Britney 2.0 - S4-E2
Continuity mistake: In Britney 2.0, when Brittany and Santana are video calling, look at Britt's laptop. Don't look at Santana, look at Brittany. In the first shot, there's some kind of light probably a window, at the back of Brittany. In the second shot, there's no light or window at all. (00:04:10)
Continuity mistake: While Jake is singing to Marley in history class, the clock drastically changes times from one shot to the next.
Factual error: The wedding is at a Christian church performed by a Christian minister, but at a Christian wedding the groom stands on the Minister's left side and the bride on his right. The way Will waits for Emma is the way that the bridal party stands at a Jewish wedding.
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.