All pupils at Redhill Academy in Arnold, Nottinghamshire, must wear trousers and skirts with the school logo on from September.
It said some clothes sold as schoolwear were not "appropriate" and pupils should look "consistent" in appearance.
But the new garments cost between £16 and £19 each, and parents say they will struggle to afford them.
Sharon Turton, who has a son and daughter at the school, said the change was going to cost her "a fortune".
"It's more than double what we are currently paying for one pair," she said.
"You don't buy just one pair of school trousers. You always buy two or three pairs to allow for dirt, damage or whatever."
The petition has been signed by about 500 people.
It states: "Not all children will conform to the sizes but most importantly the price is extortionate.
"Lower income families currently struggle with uniform costs.
"The board cannot expect these families to pay more, just for a logo."
The skirts and trousers can only be bought from one approved supplier, which already supplies the school's blazers and ties.
The school said the new skirts and trousers were better quality than items sold on the high street.
"They have already been adopted in some of our other Redhill Trust schools and feedback from parents has been extremely positive on the issues of durability and value for money," the school said in a statement.
"Retailers are not always helpful to parents in this regard by promoting a wide array of styles and fabrics as 'schoolwear' when in our opinion they are not necessarily appropriate for school for any number of reasons."
A petition against a school's "extortionate" new uniform policy has been signed by hundreds of parents.