In Glasgow you could go to a school in Bearsden and a school in Easterhouse, just a couple of miles away as the crow flies. You could note the difference in height of children of the same age. You may also note various other differences, such as motor skills and co ordination, confidence, energy levels. You could also note the fabric of the respective buildings, the resources, the staffing levels, and a long list of other things.
These differences are apparent already in primary school age children, and they will break your heart.