The concordat agreement would not stand up in court if a council tried to rely on it to refuse a placing request to a P1-3 class of 18 pupils, the Educational Institute of Scotland has claimed.
Despite asking local authorities to make "year-on-year progress" towards cutting class sizes to 18 in P1-3, the concordat had no legal standing, the education committee heard last week.
Ronnie Smith, EIS general secretary, said that if faced with a placing request challenge from a parent, a sheriff was likely to say: "What's the statutory provision?" That remained a maximum of 30 pupils per class.
The EIS called for the law to be changed or agreement to be reached through the Scottish Negotiating Committee for Teachers to ensure classes of fewer than 30 were "secure from external challenge".
East Renfrewshire Council receives around 600 placing requests every year. Its policy is to limit P1-3 classes to 25 and it is working towards the new target of 18 pupils per class in the first three years of primary.
However, if a placing request were taken "to the wire" by a parent, the authority could not refuse it, conceded a spokesman.
"Class sizes of 18 are an aspiration and so are 25, but the law is 30," he said.