Schools will have changed since you were a pupil. Try to visit at least three schools – and a good range (large and small, co-ed and single-sex, etc) to test out your assumptions and pre-conceptions. You may find, for example, that it is not the small school that provides the most nurturing environment and the local girls’ school does better at physics.
Education is essentially about people; the relationships between pupils and teachers largely determine the character and success of a school. So get a feel for what the atmosphere is like. Do staff treat the senior pupils as adults? Do pupils reflect the impression you have of the school? Are they overwhelmed with homework? Are they interested in all those extra-curricular activities? How well would your child get on with them? What do pupils like and not like about the school and what would they change if they could?