And that's a DUMB argument, since Japan isn't that stupid, no matter what he might have learned while jacking off to ecchi anime.
In truth, they are serious about education and honor, but they are also serious about crime, and in the cases of crimes that involve torture, kidnapping, or are excessively cruel or gory, they would take that very seriously and look under every rock and bush for the killer.
Besides, Japan actually is pretty low on the scale of high crime rates by nation, and their criminal justice system is very harsh. Further, given how Japanese society has a traditional strong deference to authority, criminal activity is regarded as an offense to this cultural credo and thus those prone to criminal acts tend to stand out.
In the case of a school where lots of dead bodies keep turning up, you could expect the police to burn shoe leather night and day looking for the source of the crimes.