If they had gotten together right after graduation I would guess there had been something going on the whole time and that's sketchy. If one of those teachers ended up dating someone I had gone to school with it wouldn't be a big deal. I've known some pedo-bears who've dated high school students. What kind of person finds a teenager emotionally attractive? Now if a teacher was repetitively dating students that "just turned 18" that I would completely judge.
In the UK, by law a teacher cannot date a current student of any age and cannot date an ex-student below 18 (mandatory education finishes at 16). I do have a friend who dated someone who was a student at the rival school of where he student taught, after they had both finished. They were from small town Midwest, and some of the locals tried to launch a criminal investigation. I know several people who have had relationships across the lines in college. When I was 16-18, I dated someone who was 25-27 during the course of the relationship. Principal of one of my old schools had been a girl's coach and high school teacher.
A two-year age difference isn’t particularly alarming, and dating is fairly standard at that age.
But if these teens are having sex, and you live in a state where prosecutors aggressively enforce the law, it’s possible that your son could be charged with statutory rape.
I thought it was weird and suggested the op report it to school administration because I felt like if they would proposition an ex student eventually they might proposition a current one.