The panel, who were sitting in Cardiff, were told that the teacher and the pupil exchanged messages on Facebook.
The girl was self-harming, and the teacher, Lisa Manship, was said to have become a 'shoulder to cry on'. It was alleged she ended her online messages with kisses, and also hugged the girl.
Communicating with her on Facebook was contrary to school policies at Ysgol Gyfun Cwm Rhymni in Fleur de Lys, the hearing was told.
The South Wales Argus reports the teacher had previously assured school colleagues she was not using the social networking site with the pupil, but was found to have exchanged 'foul-mouthed' messages with her between 2009 and 2011.
She was sacked from her job in November 2012 for gross misconduct .
Headteacher Owain Ap Dafydd told the teaching council panel that the pupil concerned was 'a very nice little girl'.
"She was 17 but was looking about 13," he said, "She was very needy. She was the kind of person who would look to somebody for support. It shouldn't have happened in the first place. The communication shouldn't have been there. It's just wrong.
"Everything about it is wrong, the level and quality of the language and the content, referring to self harming, all about childish gossip and children after school. I just found it terrible."
The panel was told the girl started most of the Facebook conversations, although Ms Manship initiated some of them.
The teacher had tried to stop contact on the site with the girl, the hearing was told.
Ms Manship was accused of failing to inform the school's child protection coordinator about the self-harming conversations, but she told Council members she believed the girl did not self-harm at the time she exchanged messages with her on the subject.
She denies having an inappropriate relationship and/or friendship with the girl, inappropriate conversations with her via Facebook or that the two matters amount to unacceptable professional conduct.
The hearing continues.