RESISTANCE from parents is the main obstacle facing school principals who want to switch the medium of instruction from English to Chinese. Parents would rather have their children study in English, which they believe will always be the most important language in Hong Kong. The Education Department revealed that only 33.3 per cent (or 28,510) of secondary students can learn effectively in either English or Chinese, while the rest ought to be taught in Chinese. Academic research has found that a student should reach a ''threshold level'' in use of a second language before he or she can benefit from learning in that language. A spokesman of the Schools Division of the department said more and more principals were coming to realise that students were better off studying in the mother tongue, as a low English proficiency did not equip them for an education in English. Research conducted in Hong Kong found that students educated in Chinese had a better learning attitude and obtained better HKCEE results. Principals would also like to establish Chinese in schools in preparation for the 1997 handover when Chinese would become the official language. However, principals are reluctant to make the move. They fear a change in the medium of instruction would cause parents to withdraw their children, and that fewer students would pick their schools as first choice in the secondary school places allocation system. In a recent survey conducted by the Education Department, only 52 out of 392 secondary schools opted to use Chinese as the sole medium of instruction, with 57 per cent choosing English. Changing the medium to Chinese usually involves making a wide range of adjustments in the school, including finding staff who can teach in Chinese, sending current staff for Chinese-enhancement courses, translating notes into modern standard Chinese, finding good Chinese textbooks, and making up for students' loss of exposure to English by improving the quality of English classes. Since 1986, the Education Department has been introducing a range of benefits to encourage schools to switch to Chinese. These include an extra $16,000 for library funds, expense money for partitioning classrooms (that is, to split classes for intensive English instruction), installing a loop system for listening classes, and increasing the number of English teachers. ''We will gauge individual school needs, to see how many extra English teachers each school must have,'' the spokesman said. ''If a school changes 75 per cent of its curriculum to the Chinese medium, it may recruit one more graduate English teacher. Two certificate English teachers and one graduate teacher are allowed if the school adopts a complete Chinese medium curriculum.''