International research involving Scottish schools clearly shows that high class sizes lead to poorer performance, Ronnie Smith, Educational Institute of Scotland general secretary, says in a new year message that will set the union's political agenda ahead of the parliamentary elections in May.
The EIS will again press its case for smaller class sizes after evidence from the Programme for International Student Assessment (PISA) confirmed that there is a marked dip in performance where classes number over 25.
The data shows that Scotland is far down the league of countries in terms of class size, according to the union. Recent figures from the Organisation for Economic Co-operation and Development (OECD) study of international performance in the PISA report confirm that Scotland has a class size average of 25.5, compared to the organisation's average of 22.
"Countries such as Poland, Portugal, the Czech and Slovak republics, Belgium and Spain all have lower class sizes than Scotland," Mr Smith said.
He continued: "Government representatives for many years have pointed to a lack of research evidence to support the case for a reduction in class sizes. Increasingly, this research evidence is becoming available and the PISA evidence is perhaps the most powerful evidence currently available."
The general secretary said it complements work in the United States. "All current research evidence points to what teachers and indeed pupils and students already know - and that is that class sizes do make a difference."
Mr Smith said class sizes set in the 1960s and 1970s do not fit with the education service in 2003. Many primary and secondary classes in S1 and S2 were up to the contractural maximum of 33.
However, he accepted that teachers could not expect the union's objective of a class size maximum of 20 to be achieved in the short term. But the EIS was looking for political parties to make a commitment to cutting classes that would bring genuine benefits for students.