a) Because the country can't afford it.
b) Because a lot of it depends on the parents. Children who don't come from parents who prioritize academic achievement and therefore have little interest in school pull down the kids who do. Generally parents who come from affluent areas have parents who do prioritize school.
I know this isn't a politically correct thing to say but it's the truth.
Yes they can. It's not about monetary input - it's about approach and mindset.
Also, it really isn't anything to do with parents - it's a nonsense to say poor parents care less about their kids education - they simply have less choice.
Political correctness has failed the education system. There's been a reluctance to say some kids are better (and worse) than others in school, and instead they just plow on with one set curriculum that will benefit 10% of the whole school and turn off the rest.
Kids aren't allowed to "win" in PE sports anymore. They've phased out "gifted and talented" groupings because they were afraid of recognising difference.
And that's why grammar schools work, because they have the capability to say "yes, these kids are better than other kids, we can teach them to a higher level". My argument is that we simply take that mindset and put it in place in existing schools.