What the World Needs Now - S6-E6
Continuity mistake: When Rachel is singing the final long note of "Promises, Promises," she slowly raises her arms to above shoulder level. Then, a shot from behind shows her arms well below shoulder level, then a return to a front shot shows her arms slightly above shoulder level again.
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.
BaconIsMyBFF