More than 60 children were left off a database used to draw up a controversial school admissions system.
Council officers came up with a variety of models to find the school admission scheme that would work best for Brighton and Hove.
The models were supposed to be based on the numbers of children living in various areas of the city so school numbers could be balanced. But a computer error led to 66 youngsters being omitted.
All the children live in the area proposed as a catchment for Dorothy Stringer and Varndean schools, which are expected to be among the most popular schools in the city.
Opponents of Brighton and Hove City Council's catchment area proposals said the mistake was indicative of what they believe is a flawed system.
Tracey-Ann Ross, spokeswoman for the Schools 4 Communities campaign, said: "The council has admitted its numbers are wrong and there are not enough secondary places for children in the city. It now has become a farcical exercise of damage limitation."
The council admitted the children were left out of initial calculations but said they have since been included. It refuted the claim there were not enough school places as a result.
Gil Sweetenham, the council's assistant director of education, said: "We have a responsibility to find secondary school places for all Brighton and Hove children wishing them.
"In the event of there being more children than places the council will have to provide more places, as it has in the past."
The 66 children had been missed off because the data was collected on the basis of postcodes.
They were all registered under a postcode abolished two years ago and should have been re-registered under either BN2 0 or BN2 9 postcodes which replaced the BN2 2 code when it was removed to avoid confusion with a similar code elsewhere in Sussex.
The council has conducted the admissions review in an attempt to provide fairer access to school places.
The existing system allows parents to specify their first three choices of schools and, at oversubscribed schools, gives priority to families the shortest walking distance away.
But children in several areas of the city, including East Brighton, the seafront and central Hove, live so far from their nearest schools they have no chance of being given priority at any of them. Many have to travel across the city to attend schools miles from home.
The proposed catchment areas are intended to ensure every child will be able to get into one of its nearest schools but have angered parents who believe it will reduce options for their children. Mr Sweetenham said: "We must not forget that it is often easier to rubbish new ideas rather than come up with constructive alternatives."
Mr Sweetenham and other council representatives met school governors to discuss the plans yesterday.
The council schools committee will decide whether to give the scheme the go-ahead at Brighton Town Hall on Monday, January 29, at 5pm. If the proposals are approved, they could come into action for September 2008.