Eppie Sprung Dawson, 26, was caught parked in a lay-by in Dumfries with the 17-year-old boy by police officers on a routine patrol.
Dumfries Sheriff Court was told that the pair were naked from the waist down when police knocked on Sprung Dawson's car window.
English teacher Sprung Dawson, whose husband Ranald, 32, is also a teacher, admitted breaching a position of trust.
The St Joseph's College teacher was caught on the Cummertrees to Hoddom road in Dumfries on December 21.
Sheriff George Jamieson deferred sentence but told Sprung Dawson she would be placed on the sex offenders' register.
Solicitor David Finnie, defending Sprung Dawson, said she had been seen by a psychologist and a report had been prepared. She will appear before the court again for sentence on May 1.
Sprung Dawson was immediately suspended from her post at the 800-pupil Dumfries comprehensive after her arrest, which followed a school dance.
She now faces being struck off the teaching register. Dumfries and Galloway Council said: "The teacher involved is no longer an employee and, therefore, it would be inappropriate to comment."
Sprung Dawson, who married her husband in April 2010, had driven the teenager to the lay-by following the dance. At the time, online education sites suggested she was well respected by her pupils.
On a website called Rate Your Teacher, one described her as a 'complete legend'.
The pupil added: "She improved my English a lot, taught me good techniques and she made me read more. She tried to make her lessons as fun as possible."
Another pupil classed her as one of 'the best teachers EVER' and a third commented: "Miss Sprung is amazing! She was such a laugh at the panto. LOVE EE BABE!"
However, one student wrote: "Two words: bad teacher."