A repair club is a table, some chairs, a box, and an adult who is not in a hurry. That is the entire specification. I have set them up in school libraries, in a corner of a dining hall, and once on a trestle in a corridor because it was the only space anyone could spare, and the corridor one worked as well as any of them.
The box needs less than people assume. Needles in a few sizes, a decent pair of scissors that never leave the box, thread in navy, grey, black and white because those are school colours, a jar of assorted buttons, iron-on repair patches, a small iron with adult supervision, and safety pins for the ones who need a result in five minutes. You can furnish it for roughly what a class set of textbooks costs and it does not need replacing every year.
Start where uniform actually fails
Two places, overwhelmingly. Trouser knees, which go on primary boys and girls alike and go through friction rather than tearing, so they thin before they hole. And cuffs, which fray on jumpers and shirts because that is where the wear is constant. After those, in order: lost buttons, a split seam at the shoulder or the crotch, a blazer lining coming away, and a broken zip, which is the one repair I would tell you honestly is often not worth teaching a beginner. Fix knees and cuffs well and you have addressed most of what sends a garment out of circulation while it is still the right size.
Teach the visible version first. A patch that shows is quicker, more forgiving of an unsteady hand, and produces a result in one sitting. Invisible mending is a skill worth having and it is the wrong place to start, because a child who spends four lunchtimes on something that still looks wrong does not come back.
What I got wrong the first time
I ran the first ones as sewing lessons. Demonstration at the front, everyone on the same stitch, a worksheet. It was orderly and it taught almost nothing, because a class learning running stitch on a scrap of calico is doing a craft exercise. What changed it was letting children bring their own broken thing. The moment the object is theirs, the question changes from can I do this stitch to can I save this jumper, and the second question is the one that carries any weight.
It also changes who turns up. Once it stopped being a sewing lesson I got children who would never have signed up for one: the ones who liked fixing bicycles, the ones who wanted somewhere quiet to be at lunchtime, the ones whose parents mend at home and who suddenly had something they were the expert in. That last group is worth watching for. A child who arrives already knowing how to sew on a button becomes the person the table asks, which is not a role they usually get.
The part that is not about textiles
Most environmental teaching asks children to consume less of something. A mending table asks them to do something instead, and the something takes twenty minutes and ends with an object in their hands that works again. I have watched the shift in a room and it is not subtle. A jumper that was rubbish at half past twelve is property at one o'clock, and the child who repaired it now has an opinion about throwing things away that nobody had to give them.
The honest limits
A repair club will not move your waste figures much, and I would not sell it to a head teacher on that basis. Some garments cannot be mended at all: bonded synthetics, heat-sealed seams, anything where the fabric has gone thin across a whole panel rather than at a point. And a weekly club depends entirely on the adult who runs it, which makes it fragile in exactly the way school clubs are always fragile. Pair it with a parent volunteer or two if you can, and accept that some weeks nobody comes. The weeks somebody does are what it is for.