Tamping down the sarcasm

Considering all they have to endure, I wonder how often middle school teachers must censor or at least modify their sarcasm.  In one of my sixth grade science papers, I wrote, "Without oxygen, we would not be living."  The teacher wrote in the margin next to it, "good point."  This was in a paper that received an A.  Imagine what comments he must have wanted to write on D and F papers.