The National Association of Elementary School Principals (NAESP) conducted a phone survey of 755 principals in 2000, which revealed that 21% of public schools had a uniform policy; 23% had firm plans to create one. However, this only accounts for those with school uniform policies and not for standardised dress codes, which provide for allowable student dress but are slightly more lax: For instance, instead of requiring everyone to wear the same shirt, or set of shirts, they may specify "any solid collared three-button polo/oxford shirt" and leave more specific details up to the parents. In 2007-2008, 55% of public school principals said their schools enforce a strict dress code, requiring students at least wear a collared shirt and trousers with an acceptable belt to school.
Adolfo Santos, a political science professor at the University of Houston-Downtown, stated that many Hispanic communities in the United States choose uniforms because many immigrants originate from countries with schools requiring uniforms.
Sylvan I. Alleyne, Velma LaPoint, Jennifer Lee and Harold W. Mitchell of The Journal of Negro Education stated that little empirical research exists regarding how effective school uniforms are in enhancing academic performance and social environments, and that little research exists regarding teachers' beliefs regarding issues related to dress codes, especially so regarding racial and ethnic minorities. In the United States, literature regarding public school student clothing and behavior cites anecdotal viewpoints from teachers. The literature discussed opinions on faculty, staff, and other employees on how to deal with student dress issues. A 2003 article of The Journal of Negro Education said that research and reports regarding the beneficial impact of school uniforms was not conclusive. Despite the federal government of the United States issuing a set of guidelines encouraging school uniforms in public schools, as of 2003 political support varied.
In 1994, the Long Beach Unified School District, in Southern California, required school uniforms in all elementary and middle schools. This began a trend for uniforms in American elementary public schools, especially in urban school districts. President Clinton praised the idea in his 1996 State of the Union Address, saying that "if it means that teenagers will stop killing each other over designer jackets, then our public schools should be able to require their students to wear school uniforms." The adoption of school- or district-wide uniform policies (or "standardised dress codes", less rigid than uniform requirements and allowing some leeway in parameters) has been motivated by a need to counter "gang clothing" or the pressure for families to purchase upscale-label clothing (to avoid their children being ignored by "fashion cliques"), as well as to improve morale and school discipline.
By 2010, the proportion of US public schools requiring uniforms had increased from 3% in 1996 to 25%.
In Puerto Rico, the Department of Education requires all students to wear school uniforms, with only medical exemptions.