It is not a crime for an 18 year old to sleep with their teacher. However, there are certain risks associated with doing so. If the teacher is in a position of authority over the student, there could be allegations of abuse of power or coercion. Additionally, if the student is still enrolled in school, there could be disciplinary action taken by the school. The teacher could also face criminal charges if they engage in sexual activity with a student who is below the age of consent.