Why does his teacher attack him?

In the story, Mrs. Dodds (a pre-algebra teacher at Yancy Academy) attacks Percy during a school trip.
Mrs. Dodds is actually one of the three Furies. The Furies are Hades's servants; their main duty is to do the bidding of their dark lord. Because Percy is suspected of having stolen Hades's Helm of Darkness, the three Furies have been tasked with finding him.
Mrs. Dodds attacks Percy because she believes that he is the one responsible for having taken her master's Helm of Darkness. Her ultimate goal is to kill him. Before Mrs. Dodds attacks Percy, she demands that he confess to his crime. For his part, Percy has no idea what Mrs. Dodds is referring to. He thinks that she is talking about Nancy Bobofit's little accident in the fountain.
When Percy does not answer satisfactorily, Mrs. Dodds attacks him. Her intent is to kill him, but she does not succeed. Mr. Brunner (Chiron in disguise) throws Percy a pen, which turns into a bronze sword. Percy then uses the sword to attack Mrs. Dodds (whose true Fury form is now visible). With a stroke, Mrs. Dodds is defeated, and her body vaporizes.