What is the significance of the return of the 11 plus?
When May was at school all state schools were secondary modern, technical or grammar. You took an exam at 11 to decide which route you would take, grammar being the academic route.
Predictably most children going to grammar school already came from advantaged backgrounds. Today those families would be moving into the catchment area of good schools/going to church/paying for private education. Theresa May, being from a church family, might be given a special bursary to attend a private school. There are still a few grammar schools and they are dominated by middle class families and many children are tutored for years to pass the exam.
In their defence, a few children from poor backgrounds do and did achieve more because they were able to attend a grammar school. However, children attending secondary moderns do and did get a far worse education that they would if they went to a school that accepted all abilities.
Back in the fifties and sixties this probably didn't matter. There were plenty of jobs for people who hadn't done well at school and they needed to be filled. Britain even encouraged immigration from the former empire to fill the jobs.
However, the evidence shows that the grammar system has never been able to effectively provide the population as a whole with a better standard of education than the current more common 'comprehensive' mixed ability system. In fact, it reduces social mobility and in general people do worse under a grammar system.
However, some people nostalgically love grammar schools and would like to see them return in the face of all the evidence. It's Trumpidt because it's a populist policy that has no foundation in fact.