In early May 2000, Philadelphia's public school district voted to require student uniforms.  The policy doesn't seek to clothe all 217,000 students in the same outfit, instead letting each school decide how pupils should dress.  This was the first big-city school district to introduce the policy universally, following a nationwide trend that seeks to reduce campus violence, break up cliques and limit adolescent isolation. 

Philadelphia has the nation's fifth largest school district.  80% of Chicago's public school students wear uniforms, as do 60% of Miami's.  According to the National Association of Elementary School Principals, 52% of principals reported an increase in student performance after introducing school uniforms.