More than half a million families in England find out on Tuesday about their children's secondary school places for this autumn.
National Offer Day comes against a backdrop of rising pressure on places.
The head of the Mumsnet website warned the admissions systems was becoming "seriously creaky" in some areas.
Schools Minister Nick Gibb said the government was "investing billions of pounds creating new schools and new school places".
The admissions process will see about 554,000 11-year-olds across England finding out whether they will be offered one of their preferred school places.
The population bulge that has seen primary schools having to expand has now reached secondary schools - and there will be concerns the rising pupil numbers will mean even tougher competition for the most sought-after schools.
The Labour Party, which wants councils to have more planning controls over creating new places, says last year 70% of local authorities saw an increase in the number of parents whose child did not get their first choice of school.
Justine Roberts, chief executive of the Mumsnet parenting website, says there were particular pressures in some parts of countries, such as London, Birmingham, Bristol and East Sussex.
"Stories abound of some families cheating the system, which only adds to people's anxiety and sense of injustice," she says.
The New Schools Network, which promotes free schools, has published research showing how the configuration of local schools can also affect parental choice.
In 62 constituencies, the study says, a "majority of the best schools on offer are faith-based schools", which means a wider choice for parents wanting a faith school, but less for those who do not.
New Schools Network director Nick Timothy said faith schools were "delivering exceptional education for many pupils" but there needed to be choice for parents wanting a "non-religious alternative".
What can you do if you don't get your place?
Be careful what you say to your child - do not say: "You are not going to that school," because, in the worst-case scenario, they might have to
You are entitled to go on to the waiting list of any school you put down on your secondary-school application form
You can appeal. There is no charge for this, and parents do not need a solicitor. However, you will need documentary evidence to back up the reasons for your appeal
Through the appeals process, parents need to show the negative impact of the child not getting a place at that school outweighs the negative impact of the school having to take an extra pupil
Bernadette John, from the Good Schools Guide, says some parents, "if they're in a really desperate situation", even consider moving to a different area before September. The pressure on places is predominantly in the big cities - London, Manchester, Liverpool and Birmingham - and with more rural areas often having spare places, moving can be one (albeit difficult) way of solving the problem
The National Union of Teachers has called for councils to be able to open schools in response to local needs and has accused the government of creating a "school-places crisis".
But Jonathan Simons, head of education at right-leaning think tank Policy Exchange, said the "principle of school choice" should not be "fashionably dismissed".
And he said parents wanted to express these personal choices more than taking a place at the school closest to home.
But there are big regional variations in admissions, showing London is not typical of the rest of the country.
A study of last year's applications suggested two in three London schools were oversubscribed - rising to 80% in some boroughs.
But across England, most schools have more places than applications.
Labour's shadow education secretary, Lucy Powell, has attacked the government's "free market approach" to creating new school places and said "families deserve a better approach to planning".
The government says it put £5bn into creating new places in the last parliament and would spend another £7bn over the next six years in response to the steep rise in pupil numbers now affecting both primary and secondary school.
Mr Gibb said: "We want every parent to be able to send their children to a good local school. Despite rising pupil numbers, the vast majority of parents are able to do so."
BBC Education article