Dozens more children could start applying for places at Brighton and Hove secondary schools.
The reorganisation of middle and first schools in the neighbouring Adur district means children will be moving on to secondary school at 11 instead of 12.
Most are expected to progress to King's Manor Community College in Shoreham, which will be extended to take in a younger year group, but parents in neighbouring Brighton and Hove have spoken of their concern many will opt to apply for places at the city's schools instead.
They believe the extra numbers could lead to children who live closer to the schools missing out.
Schools 4 Communities campaigner Tracey-Ann Ross, who stood for election to the city council last week, claimed King's Manor was not performing as well as its two nearest schools, Portslade Community College and Hove Park School.
She said: "There is little doubt some parents in West Sussex will begin to opt to send their children to the better performing schools in Brighton and Hove."
But King's Manor headteacher Heidi Brown said this was not so as her school had equally good results.
Parents have the right to apply for places at schools in neighbouring education authorities under the Greenwich ruling established in 1989.
It has been used for many years by families in areas like Southwick, Saltdean and Peacehaven to try to access places at Brighton and Hove schools which are near to their homes but within a different local authority.
The Adur district, which includes Shoreham, Southwick, Lancing and Sompting, is in West Sussex but borders Brighton and Hove.
Brighton and Hove City Council said parents' fears about an influx from the district were unfounded.
Assistant director of education Gil Sweetenham said: "People in Adur have always been able to apply to Brighton schools. We are not expecting an influx all of a sudden as a result of the change of age. In any case, the ruling stipulates we have the right to prioritise places for children from within our own authority, which we will do. No children from Brighton and Hove will miss out on places to others from Adur."
Seventeen middle and first schools in Adur are being reorganised to bring them in line with the rest of the county and country. They will make up 11 new primary, junior and first schools, due to be open by September 2008.
The current system where children move from middle to secondary school aged 12, in the middle of Key Stage 3, is considered impractical by West Sussex County Council.
A county council spokesman said: "Parents in Adur have always had the right to apply for school places in Brighton and Hove schools at the age of 11 and some have done so.
"If anything Brighton and Hove pupils will have Kings Manor as an additional option to their own schools at age 11."
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