Only if the amount of students with special needs and those in lower sets are also shown, if there is a higher population of less able people at that school, it's going to bring the overall grades down, but wont effect the students with obvious potential or goals. I was in top set everything at school, and all the kids surrounding me also had good grades, I was never held back by any students, and our teachers taught us all fine. The teachers taught the lower sets the same, but they can only teach them and help them so much.
Also those with a smaller population will be affected unfairly than those with a bigger population, 10 people failing in a small school could look exactly the same as 100 people failing in a bigger school statistic wise. Smaller schools are brought down much easier
In conclusion half and half, you have to consider the situations at the school and area