The Scottish Government has been struggling to deliver on its manifesto pledge to reduce class sizes in P1-3 to 18, since it came to power in 2007.
Education Secretary Fiona Hyslop has admitted change has been slower than she would have liked, blaming the recession, local authorities for failing to replace retired teachers and a flawed workforce-planning system.
Giving evidence to the Parliament's education committee this week, she gave the first indication that the pledge could be abandoned.
A review of class sizes in primary and secondary, led by the outgoing president of the Association of Directors of Education, David Cameron, would examine the best mechanism for setting limits, Ms Hyslop said.
This might be through legislation or teachers' terms and conditions, she told MSPs. However, the review would also "look at the merits" of setting class sizes nationally, locally or on a school-by-school basis.
"Currently, there is an unsatisfactory mix of class-size control mechanisms - regulation, circulars, teachers' conditions of service. It does not make sense to have such an assortment of arrangements in place," said Ms Hyslop.
She cited the "hotchpotch" of class sizes of 20 in S1 and S2 English and maths classes, which were set by a circular from the former Scottish Executive, and limits of 20 in practical classes which had been dictated by teachers' terms and conditions.
Similarly, while the previous administration had attempted to reduce class sizes in P1 to 25, the statutory maximum remained 30 in P1-3. However, in P4-7 the maximum was 33, according to teachers' terms and conditions.
The Scottish Government will be involved in the review along with teaching unions, directors of education, local authorities and the General Teaching Council for Scotland. It will report before Parliament's summer recess next year, and the Government intends to apply any changes from the start of the 2010 school year.