Image copyright Getty Images Image caption The government says grammars can help to promote social mobility

Grammar schools are likely to benefit wealthy families without raising overall standards, says the OECD's head of education.

Andreas Schleicher said international evidence suggested that selection was not linked to improving schools.

He said bright pupils in England were not getting enough opportunities, but grammar school tests were not reliable.

"Any kind of one-off test is likely to favour social background over true academic potential," he said.

Education Secretary Justine Greening has published plans calling for more selective schools in England, and a Department for Education spokesman said any new grammars would "prioritise the admission of disadvantaged pupils".

Meritocracy?

Mr Schleicher, the think tank's education director, was presenting the OECD's annual report comparing education systems across the industrialised world.

He said there was no relationship between increasing selection and how well school systems performed.

Image caption Andreas Schleicher says the importance of grammars is being "dramatically overplayed"

And countries such as Germany and Switzerland, where selection was widely used, were not more likely to produce high-achieving students.

"You might expect that where you have more grammar schools, you will have more of the really top students, that's not what we've seen," said Mr Schleicher.

The OECD education expert said access to selective schools was often unfairly biased towards wealthier families - and that contradicted the aim of stretching the most talented.

"I can see the case for introducing more meritocracy in the school system. Bright students here don't always have the educational opportunities they deserve," said Mr Schleicher.

Entrance tests

"But what happens in most European systems is that academic selection becomes social selection.

"Schools are very good at selecting students by their social background, but they're not very good at selecting students by their academic potential."

When admission to school was based on a one-off test, he said, "wealthy parents will find a way through it".

Image caption Education Secretary Justine Greening faced questions on grammars from MPs

But there were Asian school systems, such as Singapore and Hong Kong, that seemed to be more effective in how they selected pupils.

"They are selective, but they seem to be very good at figuring out how good students really are," said Mr Schleicher.

But focusing on grammars and selection was not the way to raise standards.

"I think the importance of grammar schools is dramatically overplayed," he said.

And there should be more investment for "more schools that are more demanding and more rigorous".

Image copyright PA Image caption Theresa May says that expanding grammars will increase the number of good school places

A Department for Education spokesman said: "We know that grammar schools provide a good education for their disadvantaged pupils, and we want more pupils from lower-income backgrounds to benefit from that."

"We are clear that relaxing restrictions on selective education can and should be to the betterment, not at the expense, of other local schools."

The annual OECD report highlighted the strengths and weaknesses of the UK's education systems.

It showed that by international standards, the children of migrants did very well - and often better than those who were not migrants.

The report says that in the UK, children born to parents born overseas were more likely to go to university than those with native-born parents.

Mr Schleicher suggested that this could reflect "higher levels of motivation" among migrants, who saw education as a "way of moving up the ladder".

In contrast, he said, British-born families were more likely to face "downward mobility" in their levels of education.

The bigger international picture, said Mr Schleicher, was of a "relentless" increase in the numbers going to university, with entry rates of 60% and above becoming increasingly common.

Highlights from this year's OECD education report for the UK include: