Ten Norfolk secondary schools will have more than a third of their Year 7 places unfilled when they open for the new academic year.
Admission figures show there are more than 1,600 spare places for children starting secondary schools, whose funding is largely decided by pupils numbers, forcing some to downsize.
Norfolk County Council said schools had been aware of the trend for some time, and had planned for the demographic dip which saw Year 7 numbers fall from more than 9,000 in the 2000s to a low of 7,800 last year. They are forecast to rise to around 9,000 from 2018.
- Click here to view a graphic showing admissions figures for all Norfolk secondary schools
The Hewett School in Norwich has filled 81 of its maximum of 180 Year 7 places, giving a 55pc vacancy rate for this cohort.
Deputy principal Rob Anthony said it had reduced the number of staff, mainly through natural wastage as people left or retired.
He said: 'It can reduce the number of options children have, and that's not something you want to do. But because we are a member of Open Opportunity we are still able to offer a wide-ranging curriculum because our children can access those courses elsewhere.'
Richard Snowden, head of school admissions service at Norfolk County Council, said this year's trends were driven by parental choice and the demographic dip, and added a school's medium-term reputation, rather than its most recent Ofsted or GCSE results, seemed to most influence parents.
He said special factors applied to two schools which are traditionally oversubscribed: Notre Dame in Norwich, the only Catholic secondary school in Norfolk, and Wymondham College, which has boarding students.
He said the oversubscription of Aylsham High was a more recent result of its strong performance.
He added that, despite the high number of unfilled places, no high schools were judged financially unviable.
The opening of the Jane Austen College in Norwich, a free school, has reduced pupil availability for other schools, but the council said its effect was not concentrated on any specific school. Its expected 140 Year 7 pupils is subject to change.
Executive principal Claire Heald said: 'They have not come from a particular area as such. They have come more from central Norwich, but they are from all around the city, not from a particular location.'
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