What did mrs trunchball do to the boy who coulndt spell what?

You're likely thinking of the character of Miss Trunchbull from the book "Matilda" by Roald Dahl.

Miss Trunchbull, the tyrannical headmistress, did not punish a boy for misspellings. She was known for her cruelty, but her punishments usually involved more physical abuse like throwing children across the room or locking them in the "Chokey" (a small, dark, and terrifying closet).

There is no specific incident in the book where a boy is punished for misspellings.