Conservative MP Boris Johnson donned a toga and joined students outside the House of Commons in protest at plans to scrap the last ancient history A-level.
Girls from Godolphin and Latymer School in Hammersmith, west London, handed the shadow education minister a petition signed by 4,000 people.
He is backing calls to halt plans by the OCR exam board to end the subject.
Urging the government to step in Mr Johnson said the course was of huge importance to Britain and its culture.
Speaking outside parliament on Monday the Henley-on-Thames MP said it was important not to lose a subject which could provide a route to university for many people.
"I think you're getting rid of another potential route. Why not leave things as they are? It's a wonderful subject, it's well taught, let's encourage more people to do it," he said.
Natasha Hausdorff, 17, one of the sixth-form students leading the campaign, said she was fighting to preserve a subject she loved and which could teach youngsters about democracy, religious confrontations and east-west relations.
In March OCR said it planned to scrap ancient history saying revised A-level courses would offer students a better range of qualifications.
Elements of the old ancient history exam would be taken into a new classical civilisation A-level which will be taught from September 2008.
But some critics argued the new course would not examine ancient history properly.
... a photo of Boris Johnson sort of toga-clad accompanies the original article.