Report it to the appropriate authorities in the school, such as the principal or the school board. They can take steps to investigate if there is any truth to it (which is highly unlikely) and also take measures to stop the spread of such false and inappropriate stories.
Ignore it. Chances are it's just a baseless rumor. There's no point in giving it any attention as it's unethical and inappropriate.
We should firmly oppose the spread of it. Talk to the people who are spreading it and tell them that it's wrong to spread untrue and inappropriate stories about high school teachers. Encourage them to focus on positive aspects of the school environment and the role of teachers in education.