Teaching to the test?

Can't really get my head around this idea that grammar schools should lower the pass mark for children from poorer or more disadvantaged backgrounds.  I can see the idea from a socialist, egalitarian point of view, but surely it destroys the whole concept of grammar schools?  If you're running a school for children who have shown they can attain a certain level of educational ability, then to accept any who do not reach that standard makes a nonsense of what you are seeking to do. Like I say, I can see the problem which those who advocate such a move are seeking to address, but I think they are going about it the wrong way - the real problem the disadvantaged face is that those with monied parents can afford to employ tutors to teach their children how to pass the exam. Providing such tutoring for all won't solve the problem - those with "involved" parents will always be at an advantage - but it would help to level the playing field.