In a speech to the Professional Association of Teachers at its annual conference in Cheltenham, Ms Millett said the span of learning needs in some classes is far too great for effective teaching.
"Surely teachers, whether in primary or secondary schools, need to be assured of as much homegeneity in their classes, as the subject matter needs, if they are to have the conditions where they can work most effectively to improve pupils' learning," she said.
She also said teachers will never be taken seriously as a profession while they "funk" the issue of rewarding quality teaching. "At present the automatic increment system rewards the excellent teachers and the teacher who is barely keeping his or her head above water equally and on the same terms. No wonder we have such problems retaining teachers five years into the profession. Is it not time to plan for a day when excellence receives its reward?" Ms Millett also warned teachers that the TTA intends to initiate a debate about pedagogy. "Educationists and national bodies have seemed to make an unspoken deal with teachers - 'we'll stay off the pedagogic grass provided we can landscape everything else'. Teachers themselves have sometimes pleaded for this, seeing pedagogy as their only area of independence and professionalism.
"But we should not be deterred by this plea, because a privately-arrived-at pedagogy is no more valid, and certainly no more professional, than a privately-arrived-at procedure for heart surgery."
Jackie Miller, PAT's general secretary, said the union supports setting, but she was concerned that it would be impractical in small primary schools. She said: "I am also worried that a child's set or stream could be used as the basis of selection to secondary school. This week PAT voted to reject selection at 11-plus."
Some form of setting for certain subjects is fairly standard practice in most secondary schools. And Liz Paver, head of First Intake school, Doncaster, believes there are a large number of "closet setters" in primaries. Year 3 children in her school, which has three classes, are taught reading and writing in three ability-based teams. She is considering extending this approach for other subjects, for example mathematics.
"Children are much more motivated when they are able to learn at a pace that suits them. I think that Anthea Millett was also hinting that the structure of the national curriculum will allow us to teach children in groups determined by their ability and not just their chronological age.
"Although many primary school teachers would deny setting or streaming their children, they will arrange their class in groups often determined by the level they have reached in a subject," she said.