Ronnie Smith, general secretary of the Educational Institute of Scotland, said in his report to the union's conference, that smaller classes must remain a priority. There must be no attempts to dilute the commitment to a maximum of 25 pupils in P1 and 20 in S1-S2 in English and maths by 2007.
"We hear calls for more flexibility; that different pupils have different needs; that headteachers should have more freedom to decide on class sizes in individual schools. We have no problem with that: that is not the issue," Mr Smith said.
"The issue is the limit on class size - the parameters within which these decisions are made. It's a bit like a speed limit. In built-up areas, the limit is 30mph - but that does not mean to say it is appropriate always to drive at 30. Sometimes much lower speeds are called for, but the top limit should never be exceeded."
He also warned that even though the Government had agreed to a fresh start on pensions issues after the election, EIS members should be under no illusions about the scale of problems. "The way forward is not to pressurise people into working to 65 by applying financial penalties if they go sooner," Mr Smith said. "It is striking that just over 800 out of 49,500 primary, secondary and special teachers choose to work beyond the age of 60 right now. And of 1,063 teachers entering retirement in 2003-04, just 92 - 8.6 per cent - did so because they had gone on to age 65."
In a coded attack on other teaching unions' highly charged descriptions of school indiscipline, Mr Smith said: "It seems to be the fashion at teacher union conferences to portray our schools as veritable battlegrounds where the generality of our young people, high on a daily diet of drink, drugs and gratuitous sexual promiscuity, engage in non-stop wanton acts of violence.
"I have to say that is not the message I hear from EIS members, nor is it borne out by research. There is no basis for believing it's safer to teach in Baghdad than in Barrhead.
"Let there be no doubt. Indiscipline is the principal concern of most teachers. But it is the incessant, low-level challenging behaviour that troubles teachers most - as it does many parents."
He called for teachers to be given a greater say in the overall discipline policies and strategies in their schools.
Parents had a role to play too. "In the current discussion around the Bill proposing new parent forums, I would suggest to some parents who are obsessed about issues of school governance and the preservation of school boards that a bigger priority is how to engage individual parents with their own children's education and behaviour at school."
Serious though the discipline problem was, he said, it would not be solved by demonising young people and "viewing them as the collective 'enemy' to be suppressed".
Echoing the message of Professor Kathleen Marshall, Scottish Commissioner for Children and Young People, a few weeks ago at the Stow Society lecture, he said: "Respect is a two-sided coin. There is no longer any automaticity about it. The age of deference has gone. All of us - teachers, parents, politicians and employers - must earn the respect of those for whom we have responsibility, and that depends on how we behave towards each other as well as towards young people."