A new information pack on how to use placing requests to select schools has been produced by the Govan Law Centre in Glasgow.
In particular, the resource highlights the appeals process available to parents if councils reject a request.
Currently, pupils are guaranteed a place at their local school, but families can choose to send their offspring elsewhere by lodging a placing request with the council.
Unless the school is full, the council is duty-bound to grant the request, but growing numbers are being refused.
That is because many local authorities have removed excess space by closing or merging schools. In addition, new legally binding class-size limits have been introduced in the first year of primary schools, restricting classes to just 25 pupils.
Sarah-Jane Kissock, a solicitor at Govan Law Centre's education law unit, said there had been an upsurge in requests for information from parents.
In particular, families are unclear whether schools have reached their limit for pupils or when exceptions can be made.
She said: "The right for parents to make a placing request is well known and well understood, but the appeals process is extremely complex and outdated and it can be very difficult for parents to navigate.
"We produced this tool-kit to help parents to put together effective appeal letters and to help them understand a bit more about the process and the jargon that is used."
The Scottish Parent Teacher Council welcomed the guide and said parents needed support and information even before they reached the stage of appealing a placing request.
Eileen Prior, the council's executive director, said: "Particular concerns we hear about are around the transfer from nursery to P1 in a combined school, which is not necessarily automatic, and also placements for siblings which, again, are not automatic.
"The key thing is for parents to have a clear understanding of these matters so that their choices and plans are made in full knowledge of the realities."
The system of placing requests was introduced by Margaret Thatcher's Conservative government in 1980 as part of moves to create greater social mobility.
However, critics argued it was also intended to create a market in state education, which would force less popular schools to close.
It meant parents who did not want to send their children to their local school were able for the first time to secure a place elsewhere, so long as there was space.
Previously, placing requests would only have been granted in exceptional circumstances.
Last summer, college lecturer Julie Wales from Dalry in North Ayrshire lodged a petition at the Scottish Parliament calling for new legislation to ensure siblings could go to the same school on placing requests.
She blamed the Scottish Government's policy on class sizes for the difficulties some families have experienced, saying legislation limiting P1 classes to 25 meant her daughters could not go to the same school.
Her petition called on the parliament to urge the Scottish Government to "protect the family unit" by amending current legislation to "ensure siblings are not refused placing requests to be schooled alongside family members".
She also wants councils to prepare for future upsurges in the population and avoid siblings being refused by primary schools by employing additional teachers.