Many children who pass the 11-plus exam have had extensive after-school help from professional tutors or family members, BBC research suggests.
Parents of grammar school children who responded to a BBC questionnaire said their children had spent up to a year being coached in exam techniques.
But some academics say coaching excludes children from poor families.
Local education authorities and grammar schools discourage parents from giving their children extra coaching.
England's 164 remaining grammar schools tend to be dominated by middle-class children, according to government indices, such as the number of children receiving free school meals.
Research for the BBC suggests one reason might be because of the lengths many parents go to get their child to qualify for a place.
The BBC commissioned NOP to question parents of grammar school children and 544 completed the questionnaire.
Of these parents, 443 said they either paid professional tutors or coached their children themselves at home.
Two hundred and fifty-seven had hired private tutors and spent an average £700 in fees. Many of the parents who responded also earned substantially more than average income.
Many children had received up to a year's coaching, studying for about one and a half hours a week outside of school time.
11-plus exams tuition 'routine'
Educational psychologist Professor Brendan Bunting has shown that children who are coached for nine months or more improve their scores at 11+ by up to 40%.
He believes coaching makes it much harder for children from poorer homes to qualify for grammar school.
"I think it is very difficult to believe in a system which is so open to the effects of coaching," he said.
Education authorities and individual grammar schools discourage parents and from providing or paying for any additional tuition for the children sitting this test.