A super selective school should be a specialist school for the ultra bright. At the moment wealthy parents are able to give their children an unfair advantage over working class children by paying for tutoring. Grammar schools are no longer a leg up for bright working class kids. Many children who attend grammar schools are not super gifted.
If we are going to have super selective schools then we need to make sure that places go to the right children. I do think that an eleven plus exam is useful to weed out those who do not stand a chance with coping with a faster pace of learning. However such a major decision should not be solely based on an exam
Maybe the work that a child produces in class, previous test results or school recommendation should be considered. Maybe as a final stage a child should be assessed by an educational psychologist to be sure that the child is ultra bright rather than hot housed. Some universities take into account the secondary school a student has attended. I feel that grammar schools should look at the background of the applicant and their school.
A superselective school should have a curriculum which is tougher than the national curriculum. Children who cannot keep pace should be transferred to a mainstream school.