Is the mother sinful for the complaint filed against a teacher who leaked an exam to one of the classes but not others?

Leaking exam questions is cheating and a betrayal. If you are certain that the teacher leaked them, there is no sin upon you; rather, it is a form of changing an evil and offering advice. However, if it is merely a suspicion, then it is not permissible to accuse him of that, and you must repent, seek forgiveness, and withdraw the complaint submitted to the school, because suspicion is the most untruthful of speech.