A female teacher at a prestigious Sydney private school had been warned about her behaviour towards students before she started a sexual relationship with a 17-year-old boy.

The woman, 33, had sex with the year 12 schoolboy multiple times while she was a teacher and he a student at the elite Sydney Grammar School in the inner city suburb of Darlinghurst.

Sydney Grammar School. Credit:Michele Mossop

The relationship continued over three months last year, with the pair meeting to have sex in a store room and classroom at the school and at the woman's home.

After the boy attempted to end the relationship, the woman sent him lengthy messages describing herself as a "monster", saying she was "too happy or too stupid to see the signs" and asking "am I going to go to gaol?"