A push to relax the entry criteria for one of Sydney's top selective high schools to make it easier for more local students to enrol has been rejected, after the Department of Education said the proposal would be too hard to implement.

Sydney Boys High wanted to select 30 students into year 7 who had lower entry scores than others but lived within five kilometres of the school. Extracurricular activities, such as sport and leadership roles, would have also been considered.

But in a letter to the school's principal, Kim Jaggar, the Department of Education and Communities said a review had concluded entry to selective schools had to be done on academic merit as part of a state-wide process.

''There was no strong evidence to suggest that the potential benefits of changing the selection criteria of an individual school would be justified against the possible impact on a number of other schools,'' the letter said.

There is fierce competition to secure a spot at the school and enrolments are based on the state-wide selective schools entrance exam and boys' academic performance at primary school.

A campaign is intensifying for more high school capacity in the inner city.