Is it possible that the folded pleated(whose style is folded knife pleats) skirt and tartan patterns would still be independently invented by 21st century AD modern fashion designers and be used in highschools(for female students) even if Celtic peoples never invented them?
Yes, Tocharian textiles have been found together with mummies with woollen cloth that was woven in a very similar way to tartans, Celts weren't the only ancient people with tartans most likely.