MSPs have demanded that the SNP government brings forward detailed plans on cutting class sizes.
Tories said headteachers should control class sizes
Opposition parties claimed ministers had failed to provide a statement on a manifesto pledge for sizes of no more than 18 in primaries 1-3.
But Education Secretary Fiona Hyslop said the administration was working to deliver its policies on class sizes.
MSPs voted for a Labour amendment in the Scottish Parliament, calling for the statement to be made.
MSPs also passed a government motion recognising that "early intervention" with young people had a crucial role to play in tackling inequality, as well as a Liberal Democrat amendment to develop P1 as a "transition year" from nursery to formal education.
Speaking in parliament, Labour education spokeswoman Rhona Brankin claimed the class size pledge would require Glasgow alone to hire 397 teachers and build 186 extra classrooms at a cost of £47m.
"We believe the SNP's class size pledge is under threat and unlikely to be met," she said.
Ms Brankin accused the Nationalists of avoiding giving detail, and challenged Ms Hyslop to give a projected cost for the reduction, adding: "The SNP must have one - it's a huge commitment they have made to Scottish parents."
But Ms Hyslop said talks were currently under way with councils.
The education secretary went on: "[Ms Brankin] has to decide whether she agrees with the reduction in class sizes or not.
"Because there will be many local authorities looking to see what the Labour Party position is, and whether they support lower class sizes or not - and so far, I think the answer is no."
Conservative education spokeswoman Liz Smith said class sizes should be determined by headteachers, not government.
"That is what is wrong with some aspects of SNP policy, most clearly exemplified by its universal one size fits all application of reducing class sizes in primary one to three - even when this cannot be properly funded and when it is certainly not always the appropriate priority," she told MSPs.
Jeremy Purvis, the Liberal Democrat education spokesman, said the forthcoming Scottish budget must provide a "straight answer to the straight question" of cutting class sizes.