Audio problem: In "Silly Love Songs" during the final song You can hear Blaine singing "I can explain." but in a couple of the shots he is lip-syncing to the "I love you". Next shot is he is back to singing his solo part.
Continuity mistake: When Lauren drags Puck into the storage closet for Seven Minutes In Heaven, there are hearts on the wall which appear to be decorations for Valentine's Day. As the Seven Minutes In Heaven happened prior to Sectionals, which was well before Valentine's Day, these shouldn't be there.
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