Charlie Fisher was the husband of Head-teacher Karen Fisher, he was also the dad of Jess Fisher, Harry Fisher and Bex Fisher. He became the supply teacher but Charlie was carrying this secret round with him and it was that he was having an affair with another woman. He had to tell the truth to his daughter Jess because otherwise she would've found out herself. When Karen and Charlie got a divorce it tore the family apart, Jess started veering off the rails even more and Harry's bulimia worsened. After the divorce you don't really see Charlie any more, and in Series 6 you can clearly see Charlie was more interested in helping his son with his bulimia that he didn't really care about Jess veering off the rails or that she was living a promiscuity life.