How many public schools require school uniforms?

Question:
How many public schools require school uniforms?
Response:
In 2009–10, about 19 percent of public school principals reported that their school required students to wear uniforms, an increase from 12 percent in 1999–2000.
Also in 2009–10, approximately 57 percent of public school principals reported that their school enforced a strict dress code, an increase from 47 percent in 1999–2000.

SOURCE: U.S. Department of Education, National Center for Education Statistics. (2012). Indicators of School Crime and Safety: 2011 (NCES 2012-002), Table 20.1.

From Institute of Educational Sciences website.