The largest teachers' union says schools should not be allowed to choose their own medium of instruction - putting itself at odds with principals and parents. The Professional Teachers Union (PTU), which has more than 80,000 members, said 'division and chaos' would result if schools adopted different mediums of instruction in different years and subjects. The union issued the statement despite calls for flexibility from the Subsidised Secondary Schools Council, the Secondary Schools Heads Association and various parents' groups. The government has opened a public consultation on the review of medium of instruction and school place allocation. It ends on Saturday. Shee Shing-chung, a PTU committee member, said the union agreed with a government consultation document that says schools should be allowed to choose English as the medium of instruction only when they had proved that a certain proportion of their students could learn in English. The threshold is currently 85 per cent. But Mr Shee said the government should review the threshold before implementing the new policy. 'We can't ignore the needs of the 15 per cent of students who are less effective in learning English but are placed in English-medium schools,' he said. The union said the government should provide more resources for English-medium schools to cater for such students. More resources should also be allocated to Chinese-medium schools to strengthen their English teaching, it said. A parents' action group opposed to the consultation document delivered a petition of more than 10,000 signed postcards to the gates of the Central Government Offices yesterday. Fanny Law Fan Chiu-fun, permanent secretary of education and manpower, said she understood the wish of many parents for a school-based approach on medium of instruction. However, she also pointed out that schools might not yet be ready for such a policy.