I think there are huge variations in natural ability but I agree with the argument that given lots of support and encouragement most children will achieve at least enough to go to a good university.
Between my eldest and my second child there is only 1 academic year, and despite my second child being an August born boy I was shocked at the innate ability he has. My DD1 has to work twice as hard on everything, still top of her class but with huge effort and hard graft. My DS1 finds everything massively easy, especially literacy, he was one of those children who taught himself letters at 18 months old whilst we were struggling massively to get my 3 year old DD1 to memorise some before starting school, he learned them straight away just by overhearing. This has just continued and I know that if he does put at least half the effort his sister puts in, he will reach great academic heights.
Despite the huge variation in innate ability though, both children are doing very well at school and I do wonder very often if my DD1 hadn't had the huge support at home where she would be today. Probably somewhere in the bottom set, not in maths but certainly in literacy.
My third child is even worse, he is 4 and has found everything massively hard, he doesn't get colours, letters or numbers and I know he will find school a huge challenge. I am confident though that with loads of support he'll get there in the end. My fourth child is only 2 but looks like her second brother, he seems to grasp things very quickly. So I do believe children from the same family with the same input can demontrate huge variations.