More than 60 11-year-olds at Whitgift (Motto, He who suffers, conquers) decided on Japanese as their second language in preference to Latin this year when the school offered a five-year course leading to GCSE.
Forty-six boys are now studying the subject in one of the biggest groups in schools nationwide, and a spokeswoman for the University of London Examinations and Assessment Council said: "This is excellent news. Good for them."
Last year, the board had just 554 candidates for GCSE Japanese, while 337 studied to A-level. Many of the candidates were native speakers.
The boys at Whitgift became hooked after a taster course, and last year four lower sixth-formers took GCSE after less than two years' instruction, and gained two Cs and two Ds.
Whitgift, founded in 1596 and with termly fees of Pounds 1,745, has been linked with the Urawa senior high school to the north of Tokyo, and is planning exchange visits.
It also offers French, Spanish, German, Italian and Russian. Deputy head Stephen Coyne said: "This latest move is all about showing the boys a wholly different approach and philosophy."