DlP makes a good point- the teachers can get all the credit at these schools where a huge population of the students are getting private tuition.. so it's quite false advertising.
what the what
some schools can brag about having fantastic students who are actually having to fork out for extra tuition as their teaching methodologies aren't up to scratch and then the school takes all the credit!
People are probably going to disagree with me, but I don't think it's possible to do exceedingly well with just a tutor. From my experience, teachers at school are like the plat de resistance, and tutors the side dishes: you can have an amazing meal without the side dishes, but if you do choose to have them, they can only enhance the experience (well almost).
I think where students do really well, even if a lot came from having tutors, normal teachers do deserve the credit-- they might not all be top class, but they do give feedback, mark students' work, help when necessary and hopefully inspire their students (and I think that's pretty important in students doing well). I'm generalising here, and I know this isn't always the case, but they do the best they can for a 1:20-30 teaching ratio.
In terms of the school taking credit-- why not? Success isn't always just about the teacher, but a small yet significant part is about the atmosphere and environment of learning that the school creates. And most of the high flying schools that take credit for their students' success even if most had tutors do have this sort of atmosphere.