milktwosugars - with top grades I'm sure you understand the difference between:
a) 'there's a statistically significant correlation between height and academic attainment in x circumstances' and
b) 'people achieve academically if and only if they are tall'.
The individual experience of 3 people would only contradict the later.
irvine, crucru, mrz - the article does say they took into account 'gender, race, age, parental education and health'. Presumably with age they were granular enough to consider months not just years.
The article says that they only saw the affect in big schools (despite including schools of all sizes in the study) which suggests that it's more likely to have a social cause rather than pointing to an underlying correlation between height and academic potential. (Not certain, but more likely)
This seems plausible to me. While very high ability or very confident kids might be unaffected, there may well be a chunk of middle ability, middling popularity kids for whom the increased social status (making them like school more) and increased teacher attention is enough to tip the balance on a grade or two, and make a few of them stay in education longer etc.
The obvious thing is to look in more detail at which group of students is affected and try to figure out the mechanism. Then schools can try to take that into account. Since the article was on TES, hopefully that's what is happening.