One is the eighty/twenty rule. Some students take more time than others and, not surprisingly, the students who lower a school's test average and management metrics are the ones who consume the most time and resources.
In the light of the KIPP website (which he discusses here):
Having teachers available by cell phone after school for homework help
Now, let us consider two effects of this policy. One, the school day is a lot longer for a teacher than it used to be. Add in the rigorous extracurricular activities and the (seemingly 24 hour on call status) and at best you are creating an environment where investing more resources in teaching creates better outcomes.
But I don't think that this is going to be the main effect. If you consider Mark's eighty/twenty rule, you create enormous incentives for teachers to gently nudge out the student who calls for an hour and half to discuss homework every single evening. Even if the teachers at KIPP don't do this (a proposition of which I would be highly sceptical), I would be deeply concerned about what whether this approach was scalable.
It think it would be critical to determine if an approach was scalable before basing massive reforms off of it. I am happy for innovation and improvement in education to continue but I think that we should proceed carefully and based on the evidence. In education, for reasons that evade me, people seem to be very non-critical about the evidence.