Concerns have been raised over the number of subjects students can choose from in the latter years of secondary school, following a thinktank study.
The number of courses that pupils can take at National 4 and 5 level has reduced sharply, according to Reform Scotland.
A "minority" of Scottish state schools allow pupils to sit more than six exams, with some only offering five subjects. Critics say that this is fewer than in independent schools, although others say the situation is more nuanced as schools now offer more pathways than in the past – vocational education has become more prominent in schools, for example.
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Children whose parents can afford to send them to private school or to move into another school's catchment area will be unaffected by this "unintended consequence of the Curriculum for Excellence (CfE), Reform Scotland director Chris Deerin said.
Mr Deerin added: "We are in real danger of opening up a new type of attainment gap in Scotland – one where children who are allowed to sit eight or nine National 4s or 5s will have a distinct advantage over those restricted to five or six, regardless of the latter's ability.
"The schools cutting the number of exams on offer are typically those serving our more deprived communities, further limiting the life opportunities of children who may already be disadvantaged."
Freedom of Information requests by the thinktank revealed that, in 2016, all schools in Edinburgh, East Dunbartonshire and Dumfries and Galloway allowed pupils to take eight subjects; now, no schools in either East Dunbartonshire or Dumfries and Galloway offer eight, and the limit in Edinburgh varies between six and eight.
Chair of the Commission on School Reform Keir Bloomer – who was involved in the original design of CfE – said: "One of the purposes of CfE was to broaden pupils' education, but instead the way in which it is being implemented is narrowing it significantly.
"There is ample opportunity for pupils to combine practical and academic options when they are enabled to sit nine, eight or even seven exams, but when we narrow it down to six or five there is very little room for manoeuvre.
"Reducing the number of subject options is not a government policy. It has come about by accident; the unintended consequence of ill-conceived advice. This is the hallmark of poor management. This is a lose-lose."
A Scottish government spokeswoman said that CfE "provides significant flexibility" and allows schools to "have the freedom to design a bespoke three-year senior phase of a range of courses and qualifications tailored to meet the needs of the young people at the school".
She added: "What matters is the qualifications and awards that pupils leave school with, and not only what they study in S4.
"Almost two-thirds now leave school in S6, and last year a record proportion went on to positive destinations including work, training or further study.
"Young people also now have opportunities to study towards a much broader range of qualifications, not just at school, but also at college and through apprenticeships."
Scottish Conservative shadow education secretary Liz Smith said: "This is yet further confirmation of the very serious reduction in subject choice which is affecting so many pupils across Scotland.
"It is a disgrace that the SNP has allowed this situation to develop, most especially when the evidence shows it hits hardest for those in the more deprived communities where the attainment gap is already a major issue."