Dark Sarcasm?

Today, Pink Floyd's "Another Brick in the Wall" popped up on my iPod. During the chorus, the childlike singers make an impassioned plea for "no dark sarcasm in the classroom".

I found this to be an odd lyric. What is dark sarcasm, anyway? How does it differ from any other kind of sarcasm? Are there really different types of sarcasm? And if so, what are they?

Perhaps dark sarcasm can only be understood by those who are drunk or stoned. After all, it is Pink Floyd...