NSW primary schools are banning children from bringing loom bands - brightly coloured woven bracelets - to stop playground tears over unfair sales and swaps.

Children of both sexes have become so obsessed with the bracelets, made from tiny rubber bands, that many schools have written to parents about stopping students from wearing, trading or selling them.

At Sydney's North Balgowlah Public School, the deputy principal made use of the school newsletter to warn of particular dangers associated with fads such as looms and trading cards.

''There always reaches a point when complaints re shonky deals and sneaky tradings multiply,'' Marita Nieuwenhuis wrote.