The sex criminal defines his own crime by doing what makes him or her feel good. It could be something that we wouldn’t consider as sex, such as collecting underwear or looking through keyholes. When the action that makes them feel good crosses the line of legality, we have a sex crime.
In the case of the teachers, it’s not sex that motivates them. They could arrange that at any local bar. They would not be putting their career in jeopardy(although when I was in school, just going to a bar would have been justification for firing a teacher.) There is something about the relationship with the student that makes them feel good.