As a child which is quite some time ago now
all our uniforms were polyester as there was never any options. It's a synthetic fibre that despite it's perception of a low quality the reason it is great for school uniforms is because it makes the clothes much easier to iron or even non iron, it's much more durable so with a stands playground activity and also retains colour much better than natural fibres.
Personally I wouldn't buy it for myself but for my kids and their school uniform - totally.