I'm not arguing the fact that she was in a position of power and should be punished.

"But ten months after the pair were discovered in bed, the girl's mother found out the relationship was still going on and decided to go to the police."

As young as she may be, she is not entirely innocent. As unpleasant as the truth may be to parents, boys/girls at 13 have sexual urges too. They just don't completely understand what's appropriate yet.