Police arrested a 42-year-old high school teacher Friday after he broke into Tokyo Metropolitan Mita Senior High School and placed an explosive on the principal's desk.
The explosive went off at around 11:40 a.m. while a metropolitan police bomb disposal unit was conducting a freeze-disposal. No one was injured, police said.
The suspect, Kotaro Matsumura, a mathematics teacher at Tokyo Metropolitan Hiroo Senior High School, told police that he had been planning revenge since he had been discriminated against by his colleagues.