I bought a mix -
Grey items - M&S. They were definitely better quality and came out of every wash looking perfect. They were always outgrown rather than worn out.
Sundresses - M&S. Same again. They also went to the charity shop once outgrown.
Blouses/polos - wherever was cheapest. No point spending any more, as they would be permanently stained with poster paint, ink, grass, mud, sand and mystery substances within 30 seconds of walking through the classroom door.
Tights - M&S. They could take a few kneeslides before needing repair compared to the cheaper brands and seemed to stay up better.
In Secondary, I stuck to M&S for blouses and trousers (they weren't keen on wearing skirts because of boys being arseholes) because the sizing seemed to fit them better - the other shops tended to have things that were far too large for their height - I am sure that the blouses needed less ironing, too.
I would wait until there was a discount on schoolwear and buy it all at once, so it wasn't as much of an additional cost.
My main objection was to the compulsory logo items. They have always been appalling quality, feel absolutely revolting, fit nobody and cost a fortune. So I suppose I also chose to buy M&S because if they were stuck with horrible nylon for everything else, the least I could do was buy them things that felt nicer directly against their skin.