Because as a school student, I personally think that wearing a uniform decreases the chances of getting bullied as we're all wearing the same things. Also it's cheaper and your mum has less washing to do as you wear a blazer or a jumper instead of clothes that need to be washed every single day, it's also easier in the morning aswell because, you don't have to spend so much time worrying and finding the perfect outfit for the next day. There are other ways to express yourself, not just how you dress, it could be your makeup, personality, opinion, thoughts, ideas etc.
Yes, students should be forced to wear uniforms as it brings about a sense of school identity and prevents students from being made fun of for looking differently than other students. In addition, it makes the formation of gang identity extremely difficult, thus reducing problems such as gang violence and pride.
As George Carlin said: "Don't these schools do enough making kids think alike, now they're going to make them look alike too?" We need to promote a sense of individuality and critical thinking in children, not marching them around like Hitler Youth in a conformist parade of collectivism and drone like behavior.
I do not believe students should have to wear uniforms in public schools. There are many students whose families would not be able to afford them. Or, a family may only be able to afford one and not be able to wash it every day. Additionally, wearing uniforms takes away students' ability to express themselves through their clothing.