As a current head, I can assure him that my teacher colleagues know exactly what real work is like, because they do it every day.
Didn't Mr McGowan notice his colleagues doing real work when he was a head? In fact, it is my belief that teaching children is a far more demanding job than most.
If any non-teacher reading this letter is in any doubt, I suggest they arrange to "real" work-shadow a teacher for a term-long sabbatical. They will then gain a "real" insight into the demands of the role.