Some private schools do this.
Done by the state, on a far-reaching basis, it would be done cheaply and badly.
I find it fascinating we are hearing more and more "stories" about extended, business-friendly, school hours. Poor families, getting screwed so badly. We don;t get much for our crushing, long hours, and our children being looked after by other, do we? It goes to mortgages and rents in the main. How crap.
Anyway, my feeling is that schools won't get turned into 24 hour youth holding pens. It would be too expensive to do in any way that wouldn;t raise huge safe-guarding issues.