When we label students by ability, we limit their potential to learn

Much of what you have written has my support Claire but your post lacks conviction; sorry but that is my impression. You seem to project those beliefs which do not belong to you. Rather, you write those beliefs which come across as those which your position dictates. "Sam thought she had achieved as she was expected to achieve, while I knew that she could do so much more". You 'knew' no such thing Claire; you probably had some inkling that she could do better, some suspicion based on your judgment, some gut feeling about this student; but there is a big difference between thinking something and knowing it. Unfortunately your Sam example has as much educational value as those pretending to 'know' that selection at 11 is appropriate; you see those protagonists don't 'know' either Claire.

Of course intelligence isn't the only influence on student performance. But the author is simply incorrect when she says that it is not (a) stable and (b) a very major influence on academic performance. As far as the first is concerned, Deary, Whalley, Lemmon, Crawford, & Starr (2000) found that intelligence scores were remarkably stable (0.73) over a period of 60 years within a large representative sample. And general intelligence correlates massively with performance at school (0.83 in the huge sample analysed by Cavin, Fernandes, Smith et al 2010) whilst many of the gene variants which have a sustantial influence on human intelligence (0.7 heritability) are also the ones which have a substantial influence on school performance (which has 0.51-0.81 heritability; Calvin, Deary Webbink et al, 2012). There is also a consistent literature showing that intelligence is a better predictor of educational performance than parental social class, and so it rather surprises me why policy-makers ignore this. It goes without saying that we should make every effort to make sure that every child achieves their full potential. But we also need to recognise that there are real biological differences between children which will influence their academic performance - even though many of us would wish it otherwise.