The friendship between teacher and student blossomed into an infatuation on a European high school trip in March 2006. But Leslie Merlino, then 36, knew she was headed for career-ending danger. Later, she stroked the girl's hair and began sending emails. That would lead to an assault charge and a court order to stay away from the student. From January to September 2007, they exchanged sexually explicit emails and talked frequently on the phone and in person. They held hands during strolls and met secretly in coffee shops. They kissed and fondled each other in their more private moments. By...