I know for a fact that my son wouldn't be doing as well if we weren't pushing him. We look at his books when he gets home from school to see what he's been doing and what marks he's got, we are in the same room as him when he does his homework, so we know he's doing it and we're there if he has any questions. When it comes to tests, we help him plan his revision and make sure he does it.
We do that because we both had poor educations back in the 80's. Our parents didn't care about our schooling, never checked we did our homework, barely ever went to parent's evenings. We were both "coasters" - never got into trouble, never did any revision etc, always did the only homeworks where we'd get into trouble for not doing it. The teachers just ignored us - never wondered why we were going down in terms of marks/grades as each year passed. Considering we were at schools hundreds of miles apart, our experience of schools was frighteningly similar. Result was we both got crap O levels and had to fight and work hard to get qualifications in later life to get decent careers. So, basically, two reasons for crap education - lack of parental interest and poor teaching!
Trouble is, with our son, the teaching aspect hasn't improved in the last 30+ years. From what our son says and from what we see, it's just the same. Lacklustre teachers who often don't appear to care. Our son was incredibly demoralised in year 7 when he spent hour after hour doing a history project on the crusades - he created a masterpiece, but the teacher never even bothered collecting them in, let alone marking them - a very poor example. He's had the same teacher for biology for 3 years running, yet at the latest parent's evening, the teacher couldn't remember my son by name - he had to refer to a sheet of pictures before he could recognise him - evidence that the kids who don't cause trouble and don't make a fuss are basically ignored. It's that kind of thing that have made us take control and pick up the slack that the schools won't do.
So, both parents and teachers to blame - blaming each other but not accepting their own role in the declining standards. A good teacher can make up the shortfall in parenting, good parenting can make up the shortfall in teaching, but without one or the other, it's no surprise that kids fall through the gap.