It's generally described as tongue-in-cheek because you generally have to be really bitter or outside the system to look at these things the way he does. Also, probably, because of its cheerful tone. It fits my experience of public school quite well. Those of you who have talked to me about public school are aware that I'm both bitter and outside the system at this point.
It is, regardless, a very well-written essay and a good presentation of a point of view.