To me it's the fact that he abused his position of trust to initiate a private relationship that makes me feel that a crime may have been committed. (Due to compromised consent).

Also there may be an argument to be made that a highschool teacher remains in a position of authority while a former student completes post secondary education. As the former student is likely to perceive that a former teacher has potential influence over scholarships/bursaries or want to use them as a positive reference for a summer internship after their first year or something.