I completely disagree that all middle class parents of summer born children will defer entry to gain advantage for their children. What a load of competitive parenting rubbish.
They definitely will! I was only too glad to off-load my strong-willed DD onto school as soon as it was legally possible, but if sending her on time meant that she would potentially be an even younger summer born (she's May) than she already is then I wouldn't even have to think about it. It would be a no brainer.
As it was, DH and I had agreed that if we didn't conceive the month we did, then we'd take a break to avoid a July/August baby.
I'm an August born and it definitely affected how I did in school - although more socially than academically. I have a lot of sympathy with parents of August babies who aren't ready for school and especially parents of prem babies who should really be a year below, but a blanket parental demand for all summer-borns would be a nightmare and especially for summer borns whose parents don't delay them.