I'm not suggesting that parents become teachers; I'm just suggesting that they do simple things that enhance their children's learning, that will, in the long run, help with their formal education at school. Some suggestions ...
Talk to your children (yeah, I know .... those text messages don't answer themselves .... and someone has to keep Jeremy Kyle's viewing figures up otherwise they'll be replacing him with a programme about the Australian immigration service.) Converse with them, ask them questions, challenge their ideas, take an interest in them. Read with them, take them to the library (you'd be amazed how few children I see in the library). Don't overdo it, but get them into the habit of thinking and learning. None of this requires any special knowledge on the part of the parent; it's a question of commitment.
It's unfortunate that a lot of parents see schools as being the sole deliverer of 'education' as if it's nothing to do with them.
Regarding secondary education, I would struggle to think of anything I was taught that, in itself, became of any use. Quadratic equations? I was OK at those but have never used them in the 40 years since. Secondary education is all about teaching a child how to learn, as opposed to actually imparting knowledge. Any knowledge that does become useful, does so when the pupil moves on to university or does training with their employer. (My 'O' level French might be an exception.)
Anyway, please forgive the rant above ..... it's not aimed at anyone in particular - it's just the way I see things.
And picking up one of Noah's points, a lot of low achieving children are lazy - it's just that parents don't acknowledge it and think they know better than teachers.