School uniforms are important and I say uniform not "dress code" deliberately. Uniform means uniform - one form or one style. They make wearers recognizable as part of a group, and so create a sense of identity and of belonging. They build and maintain school spirit.
Uniforms increase safety and security as wearers are quickly and easily identifiable. It is easier to locate all your students on a field trip if they all wear the same uniform. Additionally, experience has shown they reduce crime such as shoplifting, loitering and graffiti because of this identifiability factor.
Uniforms remove competition based on clothing and appearance, and any perceived need to have certain clothing and/or appearance to fit in, or to stand out. They remove the need for constant purchasing and upgrading in order to maintain a place in a social hierarchy. They remove the negativity which often accompanies having the wrong clothing and appearance, and which can quickly develop into bullying and/or self-esteem issues.
Uniforms remove conflicts over acceptability between and among students, and between students and community and students and administration. They remove First Amendment concerns and potential litigation. They symbolize the boundaries between school time and personal time.
Uniforms mask social, economic, cultural and ethnic / religious differences, not completely but enough to make a significant difference and so add to the level playing-field, individual-strengths environment we want at school.
Uniforms save money. The same five sets, or fewer if you can manage the laundry, can be worn for a very long time and significantly fewer non-uniform items are needed: after-school, weekends and holidays. Good uniforms wear harder, wash better, and require less repairing and/or replacing.
Uniforms are easy for parents and students, and choosing what to wear for school takes no time at all. Parents with tweens and teens can appreciate the significance of this.