Continuity mistake: The DVD Tina brings Mr. Chang. When she leaves she picks it up from the desk, in the last shot it is back on the desk. (00:17:38)
Continuity mistake: During "Control", Blaine's jacket can be seen repeatedly switching from closed to open. Most noticeably when the chorus shouts "oh yeah" and he's seen high-fiving another member of the group on the right of the screen. This is shown two consecutive times from two slightly different angles. The First Time his jacket is open, the second time it got buttoned up. (00:28:25)
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