A high school maths and religion teacher who had a series of sexual encounters with one of her 15-year-old male students will serve 12 months in jail, with the judge finding she had "already paid a high price for her extremely stupid behaviour’’.

The 51-year-old, who cannot be named, cried repeatedly in the dock as Downing Centre District Court Judge Colin Charteris described how she developed an inappropriate relationship with her former maths student in mid-2008 by setting up secret tutoring sessions and giving him lifts home from school.

After one such lift, the Sydney teacher let the boy touch her breasts, and later gave him her phone number.

"She was a mature woman, she knew well that she was doing wrong,’’ Judge Charteris said.

‘‘With her long history as a school teacher she was well aware that such a relationship is forbidden ... that it was not a level playing field between her and the victim.’’