He said this was confirmed by evidence from New South Wales where there is a state-wide system of literacy and numeracy tests taken at ages seven, 11 and 12 as well as external exams at 16 and 18.
"We therefore had good baseline information which we don't have in Scotland," he said. "The best predictor of attainment in the end-of-school exams was undoubtedly performance in the literacy and numeracy tests at the age of 11. But our experience in New South Wales showed that, if these tests had not been available, free meals were undoubtedly the next best thing."
Mr Clark is keen, however, to press ahead with existing work to develop prior attainment measures. But he is cautious about rushing to embrace value-added measures before they can be fully implemented and understood. He does not much like the term and prefers "pupil progress measures".
He added: "This is not just semantics. It highlights the importance of tracking pupil progress over time and taking account of the factors which impinge on that, of which the most crucial is the quality of teaching.
"If I was asked to choose the key factors in pupils' progress, I would put high teacher expectations at or near the top of the list."