The Irish National Teachers' Organisation (Into) has called for an end to the use of unregulated transfer tests by grammar schools.
It comes on the back of the recent Audit Office report on standards of literacy and numeracy, which found 9,000 pupils are leaving school without achieving five GCSEs including English and maths.
Senior Into official Brendan Harron said: "International tests show that 10 and 11-year-olds here are leading the English-speaking world in terms of reading and mathematics. Yet the NI Audit Office report shows that, by age 16, two in five pupils fail to reach satisfactory standards in English and maths. The question must be asked, what goes wrong between the ages of 11 and 16?"
According to Mr Harron, speaking ahead of the trade union's annual Northern Conference in Londonderry this weekend, the answer is simple: "Unregulated tests label the majority of our 11-year-olds as 'failures', send them to schools that are labelled as 'second class' and then wonder why these children's standards fall.
"If we really want to see the high standards at age 11 maintained and improved upon, we should end the scourge of academic selection at 11. Research has demonstrated that, in those countries where academic selection has been abandoned, more socially balanced intakes to post-primary schools has led to an improvement in standards for all pupils, with no detrimental effect on the brightest pupils."
Into's conference will be calling on the local administration to face up to its responsibilities and come up with a strategy that sees no child is left behind.