I'm not sure if it's happening more now than in the past. Maybe. When I was in the Army (National Guard) back in the late 1980s, some of the guys in the training schools (basic, AIT, etc) I attended had stories of themselves or their friends hooking up with their high school or junior high school teachers. It could be that without social media, it was less likely for the teachers to get caught.
Nowadays what sometimes happens is when the former student is old enough to go clubbing, they meet their ex teacher and bed them! I have read this occurs far more than most people realize.