John Stodter, general secretary of the Association of Directors of Education in Scotland, said that, because councils still did not have a clear legal basis on which to refuse placing requests taking P1 classes beyond 25, they would have to allow these classes in some magnet primaries to go up to 30 next session.
The Scottish Parliament's education committee heard this week that placing requests had already been submitted to councils, which would have to give decisions by April 20. In Edinburgh, hearings on placing requests begin on March 16.
This means there is no longer enough parliamentary time for the regulations to be introduced and scrutinised by the Scottish Parliament for them to take effect from the next academic session.
The failure to legislate was criticised by Marilyne MacLaren, Edinburgh City Council's education leader. She said that without a legal "back-stop" to cap classes at a maximum of 25, it was "highly likely" that 50 schools would see their P1 class numbers exceed 25.
Under usual placing request practice, applications are rejected when a class reaches 25. However, an appeal case ruling last year involving East Lothian parents means that council policy and Government guidelines alone are not sufficient to uphold a decision to deny a placing request.
This led former Education Secretary Fiona Hyslop to announce plans last September to put local authorities on a "more sound footing when making decisions regarding placing requests".
A government spokesman said: "While it is not possible to now legislate to meet the deadline for placing requests made before March 15, we still intend to consult on regulations and have them in place by the autumn.
"It is unlikely that placing requests will impact upon local authorities achieving 20 per cent of P1-3 pupils in classes of 18 or fewer, as councils will be targeting schools in areas of deprivation where placing requests are less common and class size reduction is likely to have the greatest impact."