When I was 13 I was 6ft and 180lbs. The lady in the apartment upstairs was the wife of a naval officer stationed on subs. She was freinds with my parents and needed alot of things done around the apartment that men do. Fortunately for me I was a bit slow on the nuances and her husband got a furlough. His boat put in early.

From the article at Fox, the teacher had recently broken up with her husband. Also we don't see a picture of the 13 year old boy. He may have been a big kid like I was, and maybe she made the mistake of thinking 13 ment no emotional attachment problems like sleeping with gents her own age.
Heck at 15 when I started football in high school, some of the mothers in my area were bit more freindly than the year before when my chest was smaller. I doubt age, attraction and sex fit together as neatly as some would like.