At no point in your life will you require your shoes to be replaced more than when you are at school.
There are a variety of different reasons for the need for shoe replacement, but the main one when you are a child is that your associated rate of growth is at its greatest. Everybody's feet grow at a different rate, but the rate of growth will still necessitate shoe replacement on a regular basis. It is rare that any child can continue wearing the same pair of shoes the next academic year. To make matters even better, there are a number of other factors that can prevent them from even lasting that long!
The other factor associated with being young is greater physical activity. Kids are always running about and scuffing their shoes, especially boys. How many young boys play football in their school shoes during breaks? This means that children will need replacement shoes a lot sooner than adults, although age-related factors are the main reason for replacement.
Mother Nature is an incredible thing, but there are a lot of things that need to be corrected when you are young. Many young children will tread over and the emphasis place on one side of the heel means that the shoe wears out faster than someone who doesn't have this problem. This will also depend upon the weight of the child and how badly he or she currently walks. Most parents get their children an in-sole either privately or via the NHS which helps to correct this problem. It is certainly something that should be done to prevent problems in later life, not to mention to save money on shoes in the short to medium term.
A parent may be able to get away with buying their child a pair of shoes that is half a size too big to allow for future growth, but you can't get away with much more than that as the shoe will fall off. This will create a new issue when your son returns from school with a shoe missing. It can also cause rubbing as the shoe will move up and down the child's foot which may irritate certain parts of it.
Many children will attend a games lesson only to find that their shoe is missing when they return. Sometimes kids play games and think it is funny to get rid of or hide their shoe. They don't, after all, have much appreciation of the value of money at that age. Many parents will have no alternative but to buy new shoes for their child after just a few weeks.
There is also the age old problem of buying shoes that seem alright at the time and a few weeks later you notice that your child can't walk. Naturally your son never tells you when you can return them to the shop so you have no alternative but to buy a new pair for him. Sadly, these perfectly good school shoes usually find their way into the charity shop window long before the life span is up.