A student was arrested at the Mary Lyon Pilot High School in Brighton Wednesday after being caught with a sword and knife, school officials said. The student had the weapons with him because he had martial arts practice after school, Jean-Dominique Herve Anoh, the school’s headmaster, wrote in an e-mail to parents that was obtained by the Globe Thursday. At no time were other students in danger, Anoh said. The weapons were discovered after someone noticed the student carrying a cylinder-like case, a School Department spokesman said. The school called police immediately, and the student waited for them in the headmaster’s office, Anoh said.