I can weigh in here as I can actually give a real example of this. I went from a secondary school to a grammar school. I only got to make the move because I never took my 11 plus.
I left my secondary school in yr8 because my mum found out that it of a year of 120 something students only 5 were doing anything. We constantly had science experiments cancelled because students misbehaving or doing dangerous stuff. Three students tried to saw off their fingers in tech. Another student purposefully lit a teachers hair on fire.
We had teachers trying to teach us basic maths like 1+1 because I shit you not but the teachers decided to start from the beginning so everyone could catch up. I was bearing all those in Yr 9 at yr7 in maths on stuff we hadn't been yay yet.
I was smart yes and did well but I will say, I never would have gotten to where I was without going to the grammar school. Having all those smarter people together did push you. You got to do cool science shit all the time, you could learn which is something I never really got to do at the other school (all my learning was done reading so much myself).
We should have grammar schools - it is a great way to nurture those brains but we need to help more of those naturally smart people get there.
The issue though is again and again, it is the very early years that effect that. It is home life that effects kids more than most people realise.
We love reading on our house and already our kids love books. One is 10 months but is obsessed with them. My 3 and a half year old is reading. They love books because we show them frequently how great they are. They copy us in that. At her preschool they have tapestry that allows both the preschool and parents to upload stuff. I have been uploading lots of photos and videos so they have an idea about her development (she has a habit of down playing what she can do to get praise for stuff that she finds really easy or get something she wants... She's a mini dictator I warn you). Turns out I am the only parent really using it. I'm constantly being told about how much wonderful stuff I do with her because I put her first over having a tidy home or doing what I please.
I am not smart compared to my husband or how family at all. Definitely not after all the head injuries I had as a teen. My mum is very open about how we were left in front of the TV preschool age or stayed around to just fit in around her life. No novel stories about what books they read to us as kids or anything like that. I just happened to fall in love with reading after I hung out with the smart kids from being in the higher tier stuff in primary. Turns out that it because of my dyslexia mixed with my grandad and nan doing most of the raising me part. My family lived away from my grandparents while my sister was preschool age so I had a very different preschool years from her.
We need to try and help out those younger years and those 15-30 hours will help with that but there is still so much more that could be done.
I know that I would never have gotten my grades that I have, nor the chance to go to best uni for my course in the country and I've of the best in the world staying at my secondary school. The grammar school helped me there and I'll always be thankful. I just hope now I can give my two the best start to make the most of any school they go to as I know that will make the biggest difference