The secondary school admissions process in England is still too complex for many parents, according to new research.
A report commissioned by the Research and Information on State Education trust (Rise) found that some schools are asking parents for supplementary personal information in order to choose the best pupils.
Ministers legislated on school admissions last year to prevent this practice.
But the report found that schools are using background information in order to make their decisions.
The news comes as parents on Tuesday find out whether their children got their first-choice secondary school.
Half a million families will receive admissions letters this morning, detailing whether their children will get their first preference school in September.
But Rise found that five per cent of secondary schools had selected some of their pupils based on ability or aptitude in a certain subject, an increase of two per cent since 2001.
And 17 per cent of schools were found to use religious criteria to make their selection.
Report author Anne West of the London School of Economics called for the system to be updated to make admissions fair.
She said: "You could have all the applications coming into the local authority or to a church body. On the basis of chosen criteria, someone else could then make the decision about whether a child is accepted or not.
"What you need to have is the criteria set up so that another, outside, body could make all the decisions."
But she told the BBC: "I should stress that there have been some major improvements in terms of the admissions process criteria. So children from a very disadvantaged group are given priority in line with regulations.
"But more schools, especially those responsible for their own admissions, are selecting a proportion of pupils."
West explained that the report had found attainment was being used on occasions to select pupils.
"That is not a legitimate means because basically these schools should be using aptitude," she said.
"But it is perhaps not surprising that they are using proxy ranges."
"I think the problem is that there is complexity. So you can have a high number of faith criteria which makes it complex.
"I am talking of up to 11 criteria. It might be reasonable but it still makes it complex.
"The wording creates discretion. And discretion can be used as a way of covert selection."
A significant number of schools were asking for supplementary information forms to be completed, West said.
"These should just be needed to determine religious denomination or adherence," she noted.
"They shouldn't contain personal information. They shouldn't contain marital status, the language spoken at home and so on.
"But some of them do. It is only a minority but they also ask other information that is not prohibited."
A poll for the Guardian also revealed that one in six pupils failed to get their first choice of school
The survey of 59 local authorities in England found that 100,000 11-year-olds will have to settle for their second or third choice.
Stakeholder Response: Institute of Education, London
Professor Chris Husbands, dean of the Faculty of Culture and Pedagogy at the Institute of Education, London said: "Politicians are exercised about the issue of lotteries to allocate school places. However, in many ways they have only themselves to blame: having stoked the fires of 'parental choice', the reality is that there are simply not enough places in popular schools to meet demands from all those who want them.
"There is some scope for lazy policy-making here: it might be possible to argue that the solution is to create more places in over-subscribed schools. However, in a school system which focuses substantially on choice and the importance of getting into the 'best"'school, there will never be enough places at the 'best' schools. The ideal of a good school in every community will be a chimera: as long as there is an officially sanctioned pecking order of schools, the most preferred schools will be oversubscribed.
"There is no easy or ideal way of allocating school places in such a situation. Almost every method is deeply flawed. To use geographical proximity to schools effectively links the schooling hierarchy to the housing market and preserves deep inequality. To use measures of pupil attainment to produce 'fair banding' does not solve the problem because it generally allows banding only amongst those who happen to have applied. To give priority to siblings in admission disadvantages eldest children who do not have siblings already at the school.
"Government has done a number of sensible - if sometimes ignored - things. It is right to give absolute priority to looked after children and children with special educational needs. It is right to outlaw interviews and intrusive questionnaires. But local authorities are still left with a serious difficulty: too much demand for some schools.
"In these circumstances, managed lotteries are probably fairer than almost any other process of allocation. Obviously lotteries need to be sensibly managed: it is probably easy enough to design a lottery not to split up twins, and to use a combination of residence and lottery to avoid vast school-run traffic flows.
"However, in the long run, we will only solve this problem if we downplay the assumptions around parental choice, and, instead of focusing on the apparent importance of getting one's children into the best possible school, focus on ensuring that all schools meet threshold standards, and co-operate with each other in educating the children of an area."
Dods Parliamentary Communications Ltd