Pupils were lined up at the gates of a secondary school while their trousers were checked to see if they were the right shade of grey - with some failing the inspection and being sent home.
Kepier School in Houghton-le-Spring has defended the move, which it said was because it valued "consistency".
Parents have been told that only clothes of a particular colour and bought from one supplier are allowed.
Several pupils were sent home, with others barred from classes.
Parent Kim Lister said she witnessed the checks after being alerted by her son.
She said: "I got a phone call within two minutes saying: 'Mam, they're not letting us through the school gates.'
"When I got down there were a load of children actually lined up having their uniform checked.
"I love the uniform but it would be nice if parents could have a choice where to go for the uniform."
Head teacher of the Wearside school Nicky Cooper said: "We are very, very particular about the uniform because we need consistency right across the board.
"In doing so some learners were sent home.
"If you have different types of trousers it leads on to different types of shoes, different types of shirts, etc."
Luke Bramhall, from the Children North East charity, said: "Instead of focusing on discipline and punishing students for having the right uniform, what is important is for schools to understand what each individual child and their family is going through and how they can help in difficult circumstances."
The Department of Education said schools had a duty to provide and recommend uniforms that were the best value for money.