@AgentKuga
I did a cursory search online about uniforms in England. For one, the modern UK school system is laxer compared to, let's say, the Japanese private school system that usually enforces gender-issued uniforms (i.e. men wear slacks, females wear skirts only). So that might seem a bit weird to us who are used to reading manga in the Japanese school setting.
Specifically secondary schools in the UK (i.e. middle school/high school for the US) seems to allow for more "compulsory" uniforms. That means you are free to purchase/wear any of the uniform choices provided by the school, as long as you abide by the basic dress code (i.e. have the appropriate top/bottom approved by school). This would probably explain why Alice is able to wear pants, since the College is considered a secondary school in later chapters.
So TLDR; yeah it's probably not mandatory to wear a skirt at the College.