In the TV Show Glee, why don't any of the bullies ever get in trouble for throwing slushies at members of the Glee Club?
This is a combi of two different tv tropes
A person who is cruel to others, especially those who are weaker or have less power." This sums this character up in a nutshell. They will target anyone who is less popular than they are, those who are unable to fight back, or anyone who won't fight back. Comes in different flavours as listed below.
In some shows that revolve around teenagers, preteens, or younger children, adults can't do anything right — if they appear on-screen at all. Teachers tend to be annoying sticks-in-the-mud who do nothing but spoil people's fun. Parents are clueless, no longer care, or are either over- or underprotective. And any other designated authority figures the kid might come across? Forget it.
Glee is another teen tv show, and thus revolves around teens solving their own problems, if adults did anything to punish and even expel students, there would be no tension, or show.