Laryngitis - S1-E18
Continuity mistake: During "Laryngitis", Puck pulls Mercedes up to come sing "Lady is a Tramp" with him. Yet in the very next shot, just as she starts singing, she stands up again. So she stood up twice.
Laryngitis - S1-E18
Continuity mistake: During the "Lady is a Tramp" number, Puck puts on a hat with the front of the brim flipped down. The brim miraculously flips up and back down throughout the scene.
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