Stuart Kerner, 44, from Kent, received an 18-month suspended sentence for two counts of sexual activity with a child by a person in a position of trust.
However, Pete Saunders, CEO, National Association of People Abused in Childhood (NAPAC) told Premier's News Hour he believes the judge at Inner London Crown Court has made a mistake. He said: "He's the deputy head teacher, a religious education teacher. He of all people will know that you do not cross that boundary.
"No matter what kind of attention that he may have been getting from the young person, if he couldn't deal with it then he needed to speak to a superior, not to take the young girl into the stationary cupboard and have sex with her."
Judge Joanna Greenberg QC handed Stuart Kerner, a teacher at Bexleyheath Academy, a suspended sentence because, she said, he had not actively pursued an affair with his pupil.
The judge said: "Her friends described her, accurately in my view, as stalking you ... If grooming is the right word to use, it was she who groomed you, (and) you gave in to temptation."
The judge also described the girl as "intelligent and manipulative".
The victim, who cannot be named for legal reasons, reportedly told a teacher about their affair after her parents found sexually explicit material on her phone.
The Attorney General's already launched an investigation into the judge's decision however the sentence won't be changed.
Pete Saunders, CEO, National Association of People Abused in Childhood (NAPAC):
