The girl, like many others I’ve spoken with, is not stupid, but is sadly without protection from parents
That's speculation on your part. Her mom was obviously involved enough to check her daughter's cell phone. Then she wisely contacted the police.
he is still responsible for his actions and a child is not responsible for his action.
You're stating the blatantly obvious. I didn't either say or imply that she was responsible in any way for what happened. I merely noted her immense naivete, indicated by this (from the article).....
The girl later told police she and Mulder were "just good friends," and that he “was the one person that understood and could relate to her."
Not pure speculation. The 14 year old was unsupervised enough to spend time alone with a male teacher to have a relationship with him. When I was that age - I would not have been permitted that kind of time with male teachers, coaches etc. The kid doesn’t appear to have the boundaries a protected kid would have otherwise she would have been telling her little friends that ‘lately Mr. Teacher is giving me the creeps.’ She seems to have been emotionally vulnerable and that usually is another indicator that some emotional protection was lacking. IMHO, when referring to 14 year olds, ‘Naive’ means ‘innocent and unprepared’ rather than ‘airhead’ or stupid.