Families of children with poor attendance records or special needs and refugee families will be targeted by the new "choice and transition" scheme.
The two-year £130,000 scheme, being piloted in Wandsworth, may be rolled out across England if it is a success.
It will explain to parents how to get the best out of the admissions system.
The scheme - sponsored by the Sutton Trust and the government - will give independent advice to less privileged parents about the quality, suitability and likelihood of securing a place at local secondary schools.
Officers will encourage parents to visit schools and help them understand the information supplied by the schools.
They will explain admissions processes and help parents provide the information required in application forms.
And they will advise on the financial help available for meeting the cost of school uniforms and school trips.
Parents will also be offered assistance once their child has started at secondary school.
Pupils themselves will receive extra academic help to prepare them for the Year 6 test which is sat by all pupils in the Wandsworth borough.
Sir Peter Lampl, chairman of the Sutton Trust, said: "The transition between primary and secondary schools is a critical and complex stage in a child's education.
"Many families from less privileged backgrounds do not know how to navigate it successfully and to make most of the opportunities available.
"We have trialled similar projects at Pate's School in Cheltenham and the Belvedere School in Liverpool.
"Both of these have been enormously successful, and I am sure their work can be adapted so that parents in Wandsworth and other parts of the country have access to high quality independent advice and assistance."
Scheme to be expanded
Schools Minister Andrew Adonis said the Department for Education and Skills would be evaluating the success of the pilot scheme with a view to signing up other schools and local authorities for similar initiatives.
"It is important that disadvantaged children are given the same options as other children when making that crucial move from primary to secondary school," he said.
"The chance to go to a good school should not be the exclusive preserve of only those who come from the right backgrounds."
School admissions could be confusing to some parents, especially those in "challenging" circumstances, he said.