The reason kids are leaving school unable to read and write and do simple sums is their teachers, or at least the ones who comply with government regulations long enough to stay in the game. Kids will learn as and when they're ready, and it's up to a good teacher to adapt his/her teaching, to keep them inspired and wanting to learn. If they don't get such a teacher, they're going to go out and 'learn' the stuff we'd rather they didn't, especially if their parents grew up the same way.
Schools expect working class (or 'non' working class) kids to conform to their middle class values and they show their displeasure if they won't. They expect all kids to be up to a certain standard by a certain age and if they're not, then they have to be provided with catch-up classes until they are. All kids are expected not only to achieve, but to want to achieve. Plodding along at your own pace isn't allowed. Doing your own thing isn't allowed. Comply or fail. They don't like to use the F word in teaching these days, but it boils down to the same thing, I think.
Just as an example - those of you who don't drive and don't wish to, or who have tried and decided it's not for you (if you are a driver, just imagine) - how would you feel if you were made to attend special intensive lessons to bring you up to test standard, whether you wanted it or not? How would you like to be branded a troublemaker or called lazy if you voiced your resentment? How would feel about being treated with suspicion and a lack of common courtesy by people just because you walk to work rather than drive?
My guess is you wouldn't like it much. It wouldn't do much for your self-respect for starters, and you'd have even less respect for a society that treated you like this. Yet it's OK to treat kids like it.