Uniforms vary depending on the type of school. Public schools may have more relaxed uniform standards (or dress codes) than parochial schools. One school may insist on a certain style of clothing with the school's name embroidered on a jumper or insignia. Another school may state only that certain colors and types of clothing be worn, i.e. blue tops and khaki pants and certain shoes.
If everyone wears a uniform, then no one is wearing "gang colors" or looks different than anyone else. No one can be targeted for wearing the "wrong color" or fashion. If someone at school is not dressed appropriately, it is easier for school officials to tell if that person is a stranger who does not belong on school property.
A school uniform eliminates worries about trends and fashions, so if your parents can't afford the $80 jeans that your friends have--not to worry, your friends aren't wearing them either. School uniforms "level the playing field" and can help prevent bullying or fighting over cliques and clothes.
Kids are less likely to misbehave in class when they are all dressed in uniforms. Uniforms foster better conformity with rules and standards of behavior, which are then more easily enforced. The school itself needs to have a school-wide program in place to promote all standards of behavior.
Some schools offer a "swap shop" option--parents can exchange "gently used" uniforms with other parents for size and fit. This can save families a few hundred dollars each year. Even without a swap shop option, the cost difference between uniforms over a regular wardrobe for a growing child can be several hundred dollars per year (or more).