Before I start a rant about the moral deprecation and total abandonment of whatever values there are in this world, I must say that decadence does not entail the total downfall of a civilization. It still is rather glaring though that some wizards apparently take no heed of what they are throwing away casually.
As for the first example that springs to my mind, I would point out the use of parchment. Although it adds to the exotic feeling of this world and strengthens the assumption of living in another world, I am still aghast about it all. Parchment is expensive, even if you can augment amounts of it indefinitely. You still have to raise a herd of cattle, you still have to nourish them to a certain age, then they are to be slaughtered and you prepare the skins. Parchment was expensive in medieval times, I do not expect it has gotten any better by now, nor with the advent of technology or magic. Yet, it is still used as off-hand material for casual remarks and small talk messages. And you let children write on them. There has to be something better than that. It is called paper.
A second obvious choice is to have goal posts made from gold. Wizards have a lot of free time on their hands if they do just that. Also, they have money to spend, as these goalposts are shown to be in a school, rather than a dedicated sports facility.
Naturally these are just a few symptoms of a society described by an unnamed narrator mostly focalizing via a boy 11 years old.