A Monmouthshire school has warned parents their children could be sent home if they do not wear a new official uniform.
Monmouth Comprehensive is phasing in the uniform from September and told parents they must purchase it from a preferred supplier.
Some parents are angry at having to buy £20 trousers and £16 skirts from Trutex, rather than from cheaper shops.
The school's stance is contrary to Welsh Government uniform advice.
Monmouthshire council said the school consulted with the "whole school community over a lengthy period" as per government guidelines.
Deputy head Andy Williams sent a letter to parents which said "sending students home (in line with Welsh Government guidance) is, of course, our last resort".
The community Facebook page has received hundreds of comment from concerned parents.
Paula Beddis Simpson wrote: "I am a single parent doing 2/3 jobs to make ends meet, it's totally unfair. As long as they are proper grey school trousers and pupils look smart, what is the problem?"
Alex Watkins added: "It's not as if people are refusing to adhere to the uniform, just would like to choose where to buy trousers skirts and shirts. It's hardly unreasonable."'
Year seven and sixth form pupils will be expected to wear a branded blazer and tie instead of the existing polo shirt and jumper, which the school says had become "tired, inconsistent and not fit for purpose".
Pupils in other years can change to the new uniform from September if they choose to or if their old uniform is in need of replacement
Parents claim the new uniform will cost more than £100 if they are forced to buy from the supplier.
The school said it would cost £97 for boys and £93 for girls.
The Welsh Government says governing bodies should consider stipulating "basic items and colours but not styles so that items can be bought from retail chains at reasonable prices and not just from one supplier".
The school, which will move to a new £33m building in 2018, has acknowledged "there has been some concern over the cost of trousers and skirts from our suppliers".
My Williams wrote: "We trialled the use of barcodes for parents to buy cheaper (but often less ethically sourced) items from high street retailers.
"But quickly found that codes and styles changed, allowing many students to create a very different style of uniform including tight, skinny jean type trousers and inappropriate skirts.
"The school simply cannot keep pace with brand and style changes that will add to inconsistency in a school with over 1500 students."
Council officer Will McLean said: "The design of school uniforms and the rules which apply to them are determined by school governing bodies."
Pupils eligible for free school meals will receive free uniform items in their first year.