Many grammer schools are over loaded with private school kids. Bright state school kids just can not compete. It is a massive advantage being in a class of 8 with specialist teachers and no SEN kids.
I think that the number of places for privately educated kid should be limited to the percentage of private school kids in the area. Ie. if 10% of kids in a town go to private school then 10% of places should be reserved for private school kids and 90% of places should be for everyone else.
It would then give poor state school families a chance. My son got mostly level 3s at his key stage 1 SATS and is on the top table in his class for every subject, but his school does not think he would get a place at the only grammar in the area. Its crazy. Its no wonder that social mobility is at an all time low.