Pupils at St Peter's High School in Gloucester have been given until the beginning of the next school year to smarten up, or face penalties.
Phillip Rush, the school's head, wrote in a letter to parents: "[Pupils] are expected to be in appropriate smart black or grey trousers from September and this will also be strictly enforced.
He added: "Our school's reputation is important to us and to our current and former students and we want our students to represent us in the best possible light.
"Some of our male students are wearing trousers which do not meet the uniform regulations and which are skinny."
Another senior teacher said shops were selling skinny jeans "under the guise of uniform" hinting at an illicit trade in fashionable school wear.
The dress code for girls at the school will also change next year.
Their skirts must be a minimum of 18 inches and the hem must touch the middle of the knee, Mail Online reported.
St Peter's senior vice-principal Fiona Gittins said: "It's the fashion for short skirts but it's not appropriate for the workplace and we try to instill that the school is there as a workplace and we expect the students to show themselves and the school in the best possible light."