Once a student is no longer enrolled at the school, the power imbalance inherent in the teacher-student relationship is significantly reduced. However, it's still generally considered ethically problematic and potentially even legally problematic depending on the specifics (like the age of the former student and the teacher's position of authority within the school). The potential for exploitation and abuse remains a concern, even after the student leaves the school.
Many schools have policies that address relationships between teachers and former students, even after graduation. It is best to consult the school's policies or seek legal counsel to determine any potential violations or ethical considerations in a specific situation. The most important factor is to ensure the relationship is truly consensual and free from any coercion or undue influence stemming from the prior teacher-student relationship.