And a deputy head, due to give a briefing on proposed curriculum changes, admitted that he had been muzzled.
Consultation on the new-look, skills-focused curriculum begins in January.
Ravi Pawar, from Dwr-y-Felin comprehensive school, in Neath, has been a member of an Assembly government subject advisory group since April 2005.
But he said he could only give a flavour of what was proposed because he had been told not to reveal all. He said much of the work had been looking at inclusion and what "turned pupils off" education. Consideration had also been given to whether Shakespeare should be on the curriculum for all key stage 3 pupils.
He predicted many teachers would be annoyed with assessment arrangements being revised "the wrong way round" - before the curriculum changes take effect.
Assembly government officials briefing the conference on the assessment proposals - which include accreditation of secondary schools for KS3, and primary and secondary teachers working together in school clusters on KS2 assessments - faced a barrage of questions from school leaders.
Stephen Senior, assistant head and head of science at Pen-y-dre comprehensive, Merthyr Tydfil, questioned why teachers were being asked to "accredit a curriculum that is already past its sell-by date".
John Hopkins, head of Gwernyfed high school near Brecon, Powys, said: "Is there any need for external judgements at KS3? I have yet to see a request from employers for this."
Sue Halliwell, deputy head of Ogmore school, Bridgend, said the long timeline for introduction of the assessment changes (to be completed by 2011) showed a "lack of trust" in teachers.
Robin Jones, from the Assembly government's statutory assessment team, said: "In terms of trusting teachers, this is focused on helping them with support materials, and giving constructive feedback via dialogue with the external moderator."