TOKYO — A high school student in southern Japan threw an explosive into a classroom on Friday, injuring at least 10 people, police said. The motive was not immediately clear. The 18-year-old student was arrested at the scene of the attack in Hikari public high school in Yamaguchi state, police spokesman Katsunoru Uchida said. Kyodo News agency and public broadcaster NHK said the explosive resembled a firebomb. They said around 50 people were injured, one seriously.