Mr Osler, the senior chief inspector of schools, admitted there would be "tensions" between ministers' desire for "a rigorous, purposeful national framework" and a role for councils and schools which had to deliver the policy.
But in the first opportunity headteachers have had to air their views publicly Mr Osler was challenged repeatedly over what they believed were "disempowering" and "demotivating" proposals. Brian Miller, headteacher of Dalziel High in Motherwell, said: "Many staff feel target-setting is another club to beat teachers with."
Mr Miller feared it was a step towards performance-related funding for schools.
Jean Murray, head of Shawlands Academy in Glasgow, said she had to ask her staff to achieve "absolute rather than relative" improvements in performance at a time of job losses, per capita cuts and missing pupils. In her school half of children did not have English as their main language.
David Chalmers, the head of Biggar High, said: "It is being seen as big brother not trusting schools."
Mr Osler went out of his way to calm the fears and said afterwards that schools should wait for the provisional targets, which will be issued for secondary schools in April next year, before rushing to judgment. The initiative was not "a huge culture shift" and built on existing approaches to self-evaluation and development planning.
He said firmly that "the ownership of the quality of teaching and learning must rest with the school".
"Targets are not an activity in themselves," Mr Osler said. "They are achieved through effective practice."
They were not "a punishment for backsliding on performance or parachuted in from nowhere". Target-setting was possible because of the healthy state of learning and teaching. Many schools already placed a heavy emphasis on raising attainment.
"No schools will be asked to achieve more than schools in similar socio-economic circumstances are already achieving, a quite reasonable expectation. Targets are achievable by doing what the best schools are already doing," Mr Osler said.
He pledged that the action group would ensure the emphasis on targets did not undermine the balance of the curriculum, adversely affect pupils with special needs or cause problems for pupils who were already disadvantaged.
Mr Osler also said that, while the achievement of targets was not dependent on additional resources, the Inspectorate would not hesitate to bring funding problems to the attention of ministers.
None of the heads present spoke up in support of the target-setting initiative despite Mr Osler's assurance that it was intended to be supportive and not draconian. "I have no desire to be a commissar for targets," he commented.