Education experts have delivered a mixed report card on the controversial Higher Still exams system.
The new exam system was introduced in 1999
A study by researchers said the reforms introduced in 1999 had succeeded in their aim of providing "opportunity for all".
But Professor David Raffe, of Edinburgh University, said: "This has not always led to attainment for all."
Higher Still replaced the previous system of post-16 courses in Scotland's schools and colleges.
The aim was to create a unified curriculum and assessment system offering opportunity for all.
Researchers at Edinburgh University's Moray House School of Education have carried out two surveys of data from all of the country's secondary schools and colleges.
Their work is part of a four-year study into the reforms.
Prof Raffe said the research had delivered a mixed verdict.
"It has made Scottish education more inclusive and more coherent but it has not fully achieved the 'unified system' it aimed to create," he said.
"In the sense of providing opportunity for a flexible system which all people can access, we think it has generally been a success.
"But when we look at what people have actually achieved within this system, we find a somewhat more disturbing account."
The researchers found Higher Still had improved opportunities, especially for middle and lower attaining students.
But pass rates were still much lower for students with poor standard grade results - even though the level of their courses was matched to their standard grade attainment.
Prof Raffe added: "Ten years ago we expected these differences because the curriculum students were taking wasn't sufficiently differentiated to match their different backgrounds.
"Now we should be expecting more equal levels of attainment because the courses are more tailored to where students came from and their prior attainment.
"But we're not finding that, we're finding differences of, at one end, 85% for credit level students and, on the other hand, 40% success rates for foundation students."
The study did say the new qualifications had made better progress in schools than in colleges.
Students with special needs had been given better access to national certification.
But the report concluded the overall aim of building a 'climbing frame' of learning opportunities, with flexible entry and exit points and flexible movement within the system, had proved difficult to realise.
These conclusions came as no surprise to the general secretary of the Scottish Secondary Teachers Association (SSTA), David Eaglesham.
He said: "The SSTA called for the Higher Still to be delayed by a further year, if not two years, and the minister at the time did not listen to us.
"I think we see part of the problem here in that the thing went ahead before it was fully ready.
"So it's not surprising if there's some degree of deficiency at the moment."
The convenor of the Scottish Parent Teacher Council (SPTC), Judith Gillespie, suggested further reforms might be needed with a move away from the exam system.
She said: "Passing exams is a skill and it's a skill some people have and some people don't have.
"What we perhaps need to do is to look at offering these particular youngsters routes that are exam free."
However, Education and Young People Minister Peter Peacock defended the system in a written statement.
He wrote: "We now have a qualification system which provides genuine opportunities for all pupils, from those with special needs through to the most able.
"It's also good news that schools are using the new national qualifications to deliver a more flexible curriculum."
The findings were presented at a seminar in Edinburgh on Thursday.