My daughter grew nine centimetres in one school year. By June, none of the trousers we had bought the previous August fitted, and not one of them was worn out. They went into a bag in perfectly serviceable condition. That bag is the honest picture of what happens to school uniform, and it has almost nothing to do with fabric quality.
The cost is not small. The Children's Society put the average annual spend on uniform and PE kit at roughly 400 euros a year for a primary child and 510 for a secondary one (343 and 442 pounds in the original figures), with 38 per cent of parents reporting difficulty affording it. Those are UK figures and I use them because comparable Spanish data does not exist in any form I would be willing to cite. The shape of the problem is the same here, and any parent doing a September order already knows it.
The size up trap
The obvious response is to buy big and let the child grow into it. I did this for years. It works less well than it should, for reasons that are physical rather than aesthetic. A garment that does not fit wears out faster: a sleeve rolled twice abrades at the fold, a hem that reaches the floor is walked on and frays, a shoulder seam sitting halfway down the arm puts load on the fabric rather than the seam. A trouser bought two sizes large is not being saved for later. It is being worn out early in the places you were not watching.
There is also the part nobody puts in a spreadsheet. A child who feels they look wrong will find ways not to wear the garment, and a jumper permanently in a bag is a jumper you paid for twice.
Where the growth actually goes
Children do not grow evenly and they do not grow steadily. Growth arrives in bursts, and in the primary years it goes disproportionately into limb length before it goes into width. Practically, this means sleeves and trouser legs run out long before chests and waists do, which is why a jumper is often still fine when a trouser is finished. So size to the width and buy length you can adjust: a deep hem you can let down, an adjustable waist tab, a cuff with a bit of turn in it. One size up on knitwear is usually fine because knit accommodates. One size up on tailored trousers is usually not.
Measure twice a year, in September and again in February, and write it down. Two minutes with a tape in February tells you whether you are buying in April or in August, which is the difference between planning and panic buying at full price the week before term.
Buy fewer, and plan where they go next
The other change worth making is buying less of each size and more of the sizes that last. Three shirts you wash on a short cool cycle twice a week will outlast five shirts bought in a panic, cost less, and take up less of a wardrobe. And if a garment is going to be outgrown while it is still good, decide in advance where it goes: a sibling, a school exchange, a resale scheme, a uniform bank. A bag in a cupboard is where good uniform goes to be forgotten and eventually binned.
We built measurement tracking into the parent portal for this reason and no other. If a garment is registered to a family, the size and the date are already on record, and next year's order is a comparison rather than a guess. When it is outgrown, the same record is what lets it be passed on cleanly, with its history and without carrying the previous family's identity along with it.
I am not going to end by telling you to buy better and buy less, because you know that and it is not always available to you. What I will say is that of everything in this category, sizing is the lever with the largest effect and the smallest effort. The washing advice saves a garment slowly over years. Getting the size right saves a whole garment at once.