I think you should report the school. Even the few who might (and I stress might get deported), no do something will just allow them to keep hurting people. I did some quick research and not only is +Gimmeaflakeman
right, it seems to be a criminal offense as well
[Under Article 230-1 of the Criminal Code of Japan:
“(1) A person who defames another by alleging facts in public shall, regardless of whether such facts are true or false, be punished by imprisonment with or without work for not more than three (3) years or a fine of not more than 500,000 yen.”]
Japan the only country I know of where truth is not a complete defense to defamation...