It has not happened to me, but it has happened to a friend of mine. We taught high school to adults, so my friend was teaching a student she went to high school with before this student dropped out. It’s not that big of a deal. Boundaries should be maintained. If there is any issue with a student, the teacher can have a polite conversation with the student. If it is not resolved, changes can be made to schedules, but that is a last resort. Let’s move on and get to what’s important – the student learning what is needed.
