Although nowadays a lot of public schools students ,who have various abilities and knowledge, some people believe that it is good to select by their academic abilities. In my opinion teaching to different types of ability pupils is better.
On the one hand ,some people consider that learning in high quality of groups is much better. The reason for this ,those kind of studying is perfect for understanding each other or upgrading their knowledge by fairer competitions among them . Finally they can be a brilliant scientist or master of their field in the future . For instance, in private schools whole classes full by high level children with the same knowledge. It can motivate them and force them study hard.
However on the other hand, others and in my opinion schools with mixed ability of students are better . Because this kind of students have different skills they able to assist ,giving ideas each other and make the lesson more interesting and creative. So , then year by year they will start feeling how their knowledge enhanced in this kind of atmosphere. As an example, when i studied at school in my whole class just couple of my friends know English language. Nevertheless, after seeing this me and others started interesting it and with my peers support we eventually became speaking in that language.
In conclusion , despite some part of people say the good reasons of studying with same level of schoolmates , some individuals and I also prefer to study in mixed classes