Parents will tell you that the old saw that it's not the quantity of time you spend with the kids, it's the quality doesn't really pan out. It turns out it doesn't pan out for schools either.

An Israeli researcher found that increasing effective instructional time by just an hour a week in math, science or reading can lead to improved student achievement. The extra hour made a difference, especially for girls, children of immigrant families and students whose parents had low educational backgrounds.

But there's a catch. Extra time paid off only in some kinds of schools, those schools with more autonomy to hire and fire teachers as well as make their own budgetary decisions. Interestingly, it did not pay off in schools which had been given the autonomy to pick their own pedagogical approach or choose their own curriculum.

One other noteworthy finding: extra time also paid off more if schools publicly posted their student achievement results.