The school opted out of local authority control in 1990 so that it could remain single sex.
Clare Hallows, the head, said: "We applied to the Secretary of State for Education to select one-third of our pupils because we felt that we were not getting our fair share of bright girls. Our intake was not truly comprehensive."
The school of 450 girls had been undersubscribed, but numbers have risen since the move to become partly selective.
Pupils are selected by an entrance exam in November.
Mrs Hallows said interviews might be introduced if the school became oversubscribed. "Interviews may be a good way for a parent to ensure that a school is the best place for their child. If I were a parent I should like to feel that I was choosing a school, as well as the school choosing the child."
She said there was no question of the school selecting a higher proportion of its pupils than it did at the moment even if the Government changed the rules. "We want to remain a comprehensive school."
Asked about the Government's plans to make it easier for opted out schools to go selective she said no obstacles had been put in her school's way.
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