Your last point is very important, @TeenAndTween - however we structure education, it has to make sure that every child gets the best education for them, and if a child is a late bloomer, academically, the school system needs to be flexible enough to cope with that.
However, I think we need to look at the secondary moderns, and take the good bits from them too. They provided education in practical subjects - the school I went to, which had just changed from Secondary Modern to comprehensive, was a rural school, so offered Rural Studies, to give specialist training and education to children who were going to end up working on family farms or in agriculture.
The secondary moderns offered education and training in things like vehicle mechanics, child care, construction skills - we need to recognise that some children's talents do not lie in the academic arena, and these children need education that enables them to develop their talents into marketable skills. When we try to force all children into one academic model, we let down the ones who don't fit there.
I believe that the current system leaves some children disenfranchised, and feeling at they are failures, because they are not academic. Surely it would be better if, alongside a good academic education, they were encouraged to find and develop their skills, and if we were showing them that practical skills are every bit as valuable and important as academic ones?
I don't think the Grammar/Secondary Modern system is the right way to do it, because it channels children one way or the other, with little or no flexibility. But I'm not sure we are getting it 100% right now either.