Having thrived on teaching in Chinese since the 1980s, Shun Tak Fraternal Association Tam Pak Yu College will be forced to offer three subjects in English from next year.
Principal Ho Ki-to said the school's teachers and students were frustrated by the government's change of policy on the language of instruction.
'As more and more schools will offer English classes under the new policy, we have to water down our blanket mother-tongue policy and offer mathematics, computer studies and science for junior classes [in English] next year,' he said.
Set up in 1980, the Tuen Mun school originally taught in English but decided to switch to Cantonese in phases from 1986.
'When we made the switch, we decided we would rather forgo the elite-school status and face the discrimination from parents [when they choose schools for their children] than impose English learning on students,' he said.
'Mother-tongue teaching makes for fabulous lessons where students ask questions and engage in lively discussions with teachers. They can also understand the subject matter better in their own language ... But for the sake of enrolment, we won't be able to continue our blanket mother-tongue policy in future.'
Mr Ho said a comparison of past exam results bore out the wisdom of adopting mother-tongue teaching.
'The pass rates for both English and academic subjects like history and economics have all gone up after the switch,' he said.
Form Six student Wu Ping said she felt lucky that she would not be affected by the fine-tuning policy.
'I feel sad for the young students who will have to learn more in English,' she said.
Having scored nine distinctions in the Hong Kong Certificate of Education Examination last year, the 17-year-old epitomises the success of the mother-tongue policy.
'Learning in Cantonese allows me to understand the concepts better. You have to do a lot of translation in your head if you learn in English.'
Anson Yang, principal of King Ling College in Tseung Kwan O, which also teaches all classes in Chinese, welcomed the changes.
'We will adopt 'subject-splitting' and switch the classroom language for history, geography and science from Chinese to English for our junior classes,' he said. 'For those subjects that require more complex understanding like mathematics, we would retain Chinese.'
Mr Yang said other subjects would also have more English components. 'For example, in lessons about western cooking, we can bring in more English menus and play cooking videos by such foreign chefs as Jamie Oliver.'