Even if the letter is fake, I've had similar experiences. I once got into a classroom argument with a college physics professor -- and a working physicist, no less -- who insisted that a pound was a unit of mass instead of weight/force. (The Imperial unit of mass is actually the slug, which has a weight of 32 pounds in 1 g. It's analogous to the metric unit of mass, the kilogram, having a weight of 9.8 newtons in 1 g; in both cases, the number of weight units in the mass unit is equal to the numerical value of the gravitational acceleration in the equivalent units -- 32 ft/s^2 in Imperial, 9.8 m/s^2 in metric.) What bewildered me was that nobody else in the class backed me up, even though they couldn't all have been unaware of the difference between mass and weight. In retrospect, all I can figure is that they didn't want to defy the professor. Sometimes human beings believe that following an authority figure is more important than being right.
I would bet that the child was most likely disagreeing with the teacher, so the teacher, like most adults who love wielding power over children, rather embellished the note.
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