School uniforms help students to focus on their studies, rather than worrying about fashion trends. Fashion is just one outlet for creativity. Students who wear school uniforms can be creative in other ways, like participating in creative writing, exploring their artistic sides, or being involved in music. These creative outlets can be very constructive for students.
It's a common artist's maxim that restrictions breed creativity. When students must wear school uniforms, that restriction causes them to rethink how to make themselves stand out. Dressing creatively is a rather easy way to express identity; figuring out how to use your abilities, rather than your looks, to express your individuality is more challenging and thus more rewarding later in life.
Students have many ways to express their creativity through their speech and activities at school. School uniforms keep students focused on these other outlets for creativity while simultaneously removing wardrobe competition and potential bullying targets. Students are at school to learn, not show off what clothes they have. Uniforms keep the students focused on the appropriate things.
No, school uniforms do not suppress students' creativity because students have the ability to express themselves in many other ways. Even in schools with a uniform policy, students wear jackets, jewelry, backpacks and footwear that reflect their personal style. Requiring uniforms allows administrators to keep their focus on academics and school safety, rather than on policing the length of a skirt or a pair of shorts.