Perhaps they are hoping for a reward?
You mentioned public schools -- this is off the topice but a high school student in Montgromery county, Maryland received 2 As, one in a tough honors-level geometry course. The parents protested, fearing a truant officer would come after her. Seems their daughter never attended class, was never even enrolled in the school (they put her in a Catholic school). Only the physics teacher noticed that she had not been to class and gave her a NC. So far the school has not come up with an explanation.
I wonder what the NEA had to say about it, if anything (/sarcasm).