Question to Ask the Workplace Doctors about spreading lies:
A parent in the school where I teach wrote a letter to a group of teachers which contains lies about me. How can I prove that this is slander?
You need to see an attorney about this matter. If you want to make a claim of slander or charge someone criminally with defamation, you will want an attorney anyway. (In some jurisdictions these claims are civil only and in others they can be criminal, so it varies) You can usually get a free consultation during which a paralegal or the attorney can tell you the kind of evidence you need to collect and if the lying statements rise to the level of slander. (“He’s cruel and insulting” is probably just an unfounded opinion or complaint. “He has done inappropriate things with children” would likely be defamatory.)