"Good is not good enough," Jack Barnett, president of the Educational Institute of Scotland, told delegates. Scotland was among the top performers in western Europe in educational performance but much more could be done with smaller classes.
International research showed what parents and pupils knew - that smaller classes lead to better pupil motivation and behaviour and higher attainment.
Yet in a European league table of 21 countries, Scotland sat 19th in terms of average class sizes in primary. "Only Ireland and England have a higher average," Mr Barnett declared.
Many primary and secondary classes still had up to 33 pupils, "unrealistic and unacceptable to us". Mr Barnett said: "Class sizes set in the 1960s and 1970s do not meet the requirements of teaching and learning in 2006."
A Curriculum for Excellence focused on individual pupils and a more personal approach by teachers. "With the best will in the world, how can teachers provide that level of individual attention with class sizes of up to 33 pupils?" he asked.
Mr Barnett praised the Scottish Executive for cutting class sizes in early primary and in maths and English in early secondary but called for further cuts across all classes beyond 2007 when the new limits are introduced.
Elaine Henderson, Scottish Secondary Teachers' Association, said many more teachers would need to be recruited to meet class size limits of 20 in her subject, maths, and more classrooms would be needed despite falling rolls.
* Schools should only book theatre companies that employ actors on either industry standards or union contracts, Kate McCall, Equity, said. There was a proliferation of small companies in schools but actors were often working for pound;100 a week on substandard or exploitative contracts.