The preoccupation with class sizes is "a very old-fashioned debate", according to Keir Bloomer, vice-chairman of Learning and Teaching Scotland.
Mr Bloomer told MSPs they should not worry about "how many people of the sameage you put in an identically-sized box for the next 40 minutes?".
Classes should be arranged around pupils' interests and levels of attainment, not their chronological ages, and there should be increasingly flexible class sizes.
Mr Bloomer's remarks will no doubt prompt his former union colleagues to recall that as a leading negotiator in the Educational Institute of Scotland he was a key figure in securing the teachers' contract in the 1970s which set limits on class sizes and class contact.