Lesson of the Evil (2012) on Netflix

Plot summary

An English teacher named Seiji Hasumi (Hideaki It) is loved by his students and respected by his peers. He graduates from Harvard College with an MBA, and works at Morgenstern, a European investment bank, for two years. Hasumi returns to Japan to pursue high school teaching. However, his outward charm masks his true nature. In reality, Hasumi is a sociopath who is unable to feel empathy for other human beings. Specifically, he has a severe antisocial personality disorder. Having killed both of his parents and his form tutor at the age of fourteen, Hasumi turns into a fiendishly clever killer. During his time in the States, Hasumi meets a partner in crime, an American named Dave, who thinks he shares the same "hobby" as Hasumi killing people for fun. The two can be seen carrying buckets full of human blood, bones, and organs somewhere, giving off the notion that they committed numerous murders while working together. Hasumi eventually kills his partner by knocking him out and burning him alive inside a barrel, stating that while Dave enjoys killing for fun, Hasumi does not.

Back in Japan, Hasumi chooses to deal with problems like bullying and student sexual harassment, starting by dealing with the school cheating issue. Upon collecting all of the students' cell phones prior to their exams, Hasumi secretly uses a cell phone jammer to completely prevent any cheating during the test.