What did the Trunchbull do to boy who spell what?

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

Miss Trunchbull, the tyrannical headmistress of Crunchem Hall, did not punish a boy for spelling a word. She punished a boy for being unable to spell the word "hippopotamus" correctly. The boy, Bruce Bogtrotter, was forced to eat a giant chocolate cake as punishment, which he eventually did, much to the amusement of Matilda and the other children.