So if a child cannot pass an exam, they don't get a choice? That doesn't seem very fair.
The fact is that very few parents do get a choice. If you live in an area with grammar schools, most parents of bright children will opt for their children to be educated along with other children of similar ability. The other schools in the area are secondary moderns, whether or not they choose to call themselves comprehensives.
On the other hand, if there are no grammar schools, parents will have to send their children to the same school, where bright children can do just as well. Compare the statistics I posted with Adams' Grammar School and the comprehensive, where my children went to school. They're almost identical and my children's former school has a wider range of ability, so is actually achieving higher results for its pupils, given their starting point.
My children's former school is one of the biggest in the country. It has nearly 600 in the sixth form alone. It most certainly did not put more emphasis on low achievers and most emphatically did not teach to a lower standard. It is able to provide an appropriate education for every child.