Lorna Robinson, 27, who has been in the profession for only a year, is quitting to promote Latin, Greek and classical history in inner-city comprehensives.
"Classics has all but died in the state sector," said Dr Robinson, who will leave the pound;22,995-a-year Wellington college in Berkshire this summer.
From September, she will split her time between teaching at Cheney secondary school, Oxford, and carrying out research into how classics can be better promoted in the maintained sector.
"I'm evangelical about this. I want to try to make a little bit of a difference, because unless there is a real culture change classics may disappear from the state sector. Once that happens, it will be too late to do anything about it. We need to act now," she said.
Latin and Greek all but disappeared from most state schools after the national curriculum was introduced 18 years ago. But they continue to thrive in the private sector. The OCR exam board, the only one offering GCSEs and AS-levels in classical languages, has already cut the list of compulsory words to be mastered and scrapped oral coursework.
Dr Robinson said more needed to be done. She studied classics at Oxford university and completed a doctorate in the subject at University College London last year before becoming a teacher.
She works in one of the most picturesque private schools in England - with a six-member classics department - but said she has a calling to work in the inner-city.
At Cheney, a socially diverse school where pupils speak more than 30 languages and academic performance is roughly in line with national averages, she will teach part-time, and intends to start a classics society.
She will then spend the rest of her week working in other state schools, editing a classics magazine, Iris, and doing research. "I must admit there's been a bit of a mixed reaction to this, but ultimately you have to choose what makes you happy and ensure you're not compromising yourself in anyway," she said.
"Although I like it at Wellington, I feel a bit troubled working exclusively in the private sector."
Peter Jones, co-founder of Friends of Classics, a charity funding schools who want to adopt the subject, said: "It would be a tremendous deprivation of the understanding of where we come from to lose contact with the classics. These are among the great roots of our society and they are worth preserving. Any attempt to do so is greatly welcomed."
STATE SCHOOL VANISHING ACT
Teaching of the classics has plummeted in state schools over the past 18 years.
In 1988, more than 16,000 pupils took Latin GCSE, of whom about half were state-educated. Since then the number has fallen and in 2004 GCSE entries were down to 9,886, the first time it had dipped below 10,000. A-levels have been similarly hit. Between 1965 and 2003, the number of post-16 students taking Latin fell from 7,901 to 1,310 and Greek A-level entries dropped from 1,322 to a mere 246. Lorna Robinson is compiling her own research into the state of classics teaching across England and has identified 175 schools where provision is strong. Of these, only 21 are state schools. Latin remains the overwhelming favourite among pupils. It is offered at 169 of the schools surveyed, while classical history is offered at 121 and Greek at 94 schools.