I went to a grammar- halfway through it became 'comprehensive' due to local labour government being elected I believe.
It went from being a school where bright students from humble backgrounds, got to experience learning around other students who mostly wanted to learn- to a school where you risked getting your head smashed in the wall if you answered questions during a lesson. The school fell apart slowly- teachers from the grammar started having break downs as they were ill equipped to manage the students who at that point in their lives, had less of an appetite for learning. I remember having an argument with my right on history teacher who suggested it was our job as grammar students to 'bring them up.' That did n't happen and you can imagine my response.
I ended up teaching in a grammar and then a comprehensive. This is what I observed.
1 At the grammar bright students will be supported to learn at the highest level and lessons move fast as teachers do not have to deal with managing relentless behavior issues.
2 Less bright students who only just got in may feel like the odd ones out and may not enjoy the school because the pace and level will be too much,
1 At the comprehensive your child may run up against the issue of classes moving slowly due to the many disruptions that students who find learning hard, cause.
2 At the comprehensive your child will see a greater diversity of people and you may value this over them being in a learning environment where you can really learn at the rate you can go at.
3 At the comprehensive some teachers will be very competent and will be able to manage the broader range of students and the behaviors they present- your child will have an excellent learning experience in those classes. BUT there will be either a few to many teachers ( new ones, less competent, less caring about academics) who will not manage these mixed classes, and your more academic child will have a less good experience in those classes.
I now teach in a mixed genuine comprehensive school-the brightest students get the raw deal as I move through so much less material than I did in a grammar- the difference is shocking. Your child will be competing with the students from the grammars and the private schools at university and then in life. If your child is academically able at 10/11 and you can access a grammar, I strongly advise you try to get them in,UNLESS you value them being exposed to a more diverse range of academic abilities more than them being in a learning environment where they get to learn at a pace suited to them among students who also value learning.
What I find awful is people thinking they have the right to decide that grammars should not exist due to their own political views and so people who would benefit from them, are deprived of them. If you don't agree with grammars, don't choose them but leave out the judgement of people who choose them as the right fit for their child. If you have a bright child you have a duty to make sure they get their needs met. There is more chance that will happen in a grammar school than a weak to average comprehensive. Having been in many schools I know there are plenty of weak to average ones out there, so tread carefully on your child's dreams as you guide them to the right school for them.