When your handwriting isn't all that good, as it tends not to be when you are nine, you might write "Merry Christmas" so that it looks a little like "Meany Christmas".
Then, if your teacher is the hysterical sort, who can't abide there to be any hint of unpleasantness in the world and squeals at the merest whiff of Roald Dahl, she might rip up that card, throw it in the bin, and scream "Have a Meany Christmas? What a horrible thing to write! You awful child!"
And when the child tearfully explains that it said "Merry", that same teacher - the one who strives for a perfect world in which children never have to feel pain - looks into that child's baffled and hurt eyes, sees that she has ripped up his innocence, and hopefully spends the rest of her life clawing at her own forearms like a damn maniac.