Grammar schools are dodging a ban on "pre-testing" pupils by offering potential students a mock 11-plus.
The tests are designed to show parents whether their children are grammar material and thus help them decide which schools to put on their list of preferences.
The mocks come in response to a ban on testing children before they choose schools. Schools adjudicators ruled that pupils should not be tested until after parents had made their choice because otherwise they get "two bites of the cherry" and can pick the best of the non-grammars if their children fail.
From this autumn, parents in authorities with grammar schools will have to decide on their school preferences in October, a month before their children sit the 11-plus. They will then have to wait until March to learn whether they have won a place. In most areas parents will also be expected to rank their preferences.
Martyn Morris, head of Bacup and Rawtenstall grammar in Lancashire, told a National Grammar Schools Association conference there was a danger that the changes would make grammar schools less popular.
Parents would not risk putting a grammar school as first choice if they were uncertain whether their child would pass, he said. "It's potentially a real nightmare," he said.
Mr Morris said parents had expressed strong demand for mock tests that could indicate if their child would meet a specific grammar school's requirements.
A group of grammar heads have approached the National Foundation for Educational Research to see if they could help develop 11-plus exams for this purpose, he said.
The Parents Teachers and Friends Association of Devonport high school for boys in Plymouth already runs such NFER tests each September and charges pound;2 per candidate.
Each pupil is given a code number to make them anonymous and a list of grades is then published a few weeks later alongside the codes.
This allows the families to see how their child has performed but means the school does not know which pupils have succeeded.
Dr Nicolas Pettit, headmaster of Devonport high, said around 300 pupils had sat the mock test at his school last year and that he would recommend the approach to others.
"It's run by parents for parents - the governors and I don't touch it," he said. "The critical thing is that it is anonymous."
Helen Barlow, PTFA treasurer, said the association's chief reason for holding the tests was to help future parents and that the money had enabled the school to buy a minibus and a fitness suite.
But the Campaign for State Education said it feared that grammar school "pre-tests" would exacerbate the exam stress on pupils.
Margaret Tulloch, campaign spokeswoman, said: "This is yet more evidence of the problems which selection creates for children."