Secondary schools (aged 11-16) are of school uniform, some girls wear a plain skirt, some uniform is a tartan skirt.
Some Secondary schools are called "college" in their title due to the extended college courses in the syllabus, now it is for students aged 11 to 18 years old.
Every secondary school had adopted the sixth form college age students now (17-18 years old) with a uniform code.

thanks for the info, I think most of the colleges in the world don't ask their students to wear uniform, but I was curious if the younger ones needed to put them on for school days in U.K.

In the UK we have uniform for Secondary school (Comprehensive/High School) but not for college.
The skirt that Kate wears is not typical of school uniform either. A lot longer than most you'll find here.
"Two for flinching!"