The girl, who cannot be identified by court order, said the teacher, 52, had been a father figure and mentor.
When the teacher ended their relationship, she said the pain was unbearable and felt like daggers going through her. She had considered suicide.
The girl reportedly hated not knowing whether to call him her boyfriend or just a friend, because she couldn't call him teacher or father.
She felt exposed at school because of rumours about her relationship with the teacher and "couldn't look anyone in the eye any more".
She was bullied but ignored her classmates and tried to be quiet in class so she wouldn't be noticed.
The teacher has pleaded guilty to five counts of committing an indecent act with a child aged 16 to 17 and two counts of sexual penetration of a child aged 16 to 17 between December 2010 and June last year.
The maximum penalty is 10 years jail.
Judge Phillip Coish banned the media from naming the teacher or the school in Melbourne's north-east because it could identify the girl.
Defence lawyer Anthony Lewis said the teacher was concerned if he was named then the girl could be identified and this would have an affect on her Year 12 studies.
Prosecutor Louisa Dipietrantonio told the court the victim had not wanted the relationship to end because she loved the teacher.
Mr Lewis said the teacher unreservedly apologised for breaching the girl's trust and allowing the relationship to go beyond the normal teacher-student relationship.