The proposals are the result of a month-long consultation exercise on the introduction of small-class teaching in 2009 - a key pledge announced in Chief Executive Donald Tsang Yam-kuen's policy address in October and to be discussed by Legco's education panel on Friday.

The bureau's paper states that 70 per cent of primary schools have asked to be allowed to adopt the small-class mode, under which they will be allotted 25 students per class, in next year's intake, but that some schools may not be able to go ahead with the policy because of a predicted lack of places in certain districts.

Schools going ahead with 'large' classes will be allotted 30 students per class. However, the bureau advocates giving schools a 10 per cent 'buffer', allowing them to admit extra pupils outside the central allocations process - a common practice among popular schools.

Cheung Man-kwong, president of the Professional Teachers' Union, said the cap should be rigidly set at 25 and 30 students for all schools unless they had 'special reasons' for admitting extra students. 'If you allow schools allocated 25 students to take in more, then that betrays the meaning of small-class teaching.'

Wong Kwan-yu, chairman of the Hong Kong Federation of Education Workers, said his union also opposed giving schools the option of admitting extra students. 'We are totally against this approach,' he said.

He said schools should only be allowed to admit more than 25 students if there was a shortage of places within their school net.

'If you let them take 27 students, then I don't see any difference between that and the so-called 'large' classes with 30 students,' Mr Wong said.

Alex Cheung Chi-hung, chairman of the Aided Primary Schools Heads' Association, said the policy decision appeared to be aimed at accommodating 'a minority of schools'.

'The majority of principals in our association are in favour of small-class teaching and want the cap to be fixed at 25 or 30,' he said.

But he added: 'It is true that a minority of principals still want flexibility to admit extra students. I think allowing them to take two to three more is reasonable and will not make a huge difference.'

The allocations mechanism for this year's entry will give schools either 30 or 35 students per class, depending on their teaching approach - both figures are two fewer than previous years.

There is currently no upper cap, meaning schools can admit as many as 45 per class as per fire regulations.

However, the declining student population - the result of a birth rate that fell by more than 35 per cent between 1994 and 2003 - has meant many schools are already operating small-class teaching by default.