What I guess at while going round teaching schools is that eight or nine in ten students performing well are the ones of good school. Teaching good students is very beneficial to me because they care a lot and work hard.
On the other hand, students of modest school are modest as well. First of all, they are not proud of being there. I understand it well and it's the culture of such schools. Of course I can see good students in such schools, but it's quite rare. I understand that many parents want their children to go to good school. However, it's true that even good school never guarantees their success. Good people will succeed. Alas, let bygones be bygones. My conclusion is, school doesn't matter.