The headteacher at St Peter's High School has told parents some male pupils have been breaking current uniform rules by wearing skinny trousers in the classroom.
Philip Rush has said in an open letter that only "appropriate smart black or grey trousers" will be acceptable from September this year, warning new regulations will be "rigorously enforced."
Mr Rush wrote: "School uniform is important in promoting belonging and equality.
"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."
Girls are also being warned skinny trousers will not be tolerated from the autumn term, while skirts must be at least 18-inches long and the hem must touch the middle of the knee.
In a statement on it's website, St Peter's says: "Our mission is to provide a rigorous and broad education within the context of a Christian environment.
"An understanding of the Roman Catholic faith, in line with the Gospel message, underpins the whole work of the school."