Can you take legal action against a teacher for talking bad about you to other parents?

Slander is a determination that must be made by a court. Before you take any action you should make an appointment to discuss the situation with the school principal. The principal should investigate the situation and report back to you. If that doesn't yield satisfactory results then you should call the superintendent of schools. If that doesn't yield satisfactory results you should consult with an attorney. You should collect all the facts, dates of meetings and any evidence that proves your claim and take it all with you to your consultation.