MerryM, did your letter not give you a breakdown of the different grades for each different paper? We got a full set of results which I still have (not sharing, though) The papers are indeed above GCSE, closer to AS/A Level but boys don't have to be tip top in everything to succeed - the exam is set to test ability, thought process and expression.
Indrid is right, miss the Assessment and the only route then is Scholarship, miss out on the Scholarship and there's no place. I know someone who missed the Assessment and tried for a Music Scholarship but although he managed a Music Exhibition, it wasn't quite enough to get the place. Tigermam's DS might have got a Conditional Offer had he done the Assessment but as competition for places is so hot they don't bend their own rules - it's only fair.
Btw, they don't need both Greek and Latin - they choose which papers they do. My DS didn't do the humanities tri-partite paper and doesn't do Greek anyway.
Passing the KS is a success even if they don't get (or take up) the scholarship - they don't need to do the CE. My DS was somewhere in the top 20 (s'all I'm saying) and says he's glad he didn't get into College; the demands on the KS boys are far greater than on the others, and they are expected to be the best in everything. When they're not, there is a small amount of humiliation, I think...and he was none too keen on the religious bits, although he does do College Chapel.
I think it's the boy himself who earns the place - pushy parents cannot get their boy in with excess tutoring. It's their own drive and determination to succeed that gets them through the KS (or other scholarship exam). I always got the impression that what Eton were looking for could be learned but not taught!