American Federation of Teachers, embraced the concept in 1988, when he called for the reform of the public schools by establishing "charter schools" or "schools of choice." Gloria Ladson-Billings called him "the first person to publicly propose charter schools." At the time, a few schools already existed that were not called charter schools but embodied some of their principles, such as H-B Woodlawn. As originally conceived, the ideal model of a charter school was as a legally and financially autonomous public school (without tuition, religious affiliation, or selective student admissions) that would operate much like a private business—free from many state laws and district regulations, and accountable more for student outcomes rather than for processes or inputs (such as Carnegie Units and teacher certification requirements). Minnesota was the first state to pass a charter school law in 1991. California was second, in 1992. , 43 states and the District of Columbia have charter school laws, according to the Center for Education Reform. As of 2012, an authorizer other than a local school board has granted over 60 percent of charters across the country. Between 2009 and 2012, the percent of charter schools implementing performance-based compensation increased from 19 percent to 37 percent, while the proportion that is unionized decreased from 12 percent to 7 percent. The most popular educational focus is college preparation (30 percent), while 8 percent focus on Science, Technology, Engineering, and Mathematics. Another 16 percent emphasize Core Knowledge. Blended Learning (6 percent) and Virtual/Online learning (2 percent) are in use. When compared to traditional public schools, charters serve a more disadvantaged student population, including more low-income and minority students. Sixty-one percent of charter schools serve a student population where over 60 percent qualify for the federal Free or Reduced Lunch Program. Charter schools receive an average 36 percent less revenue per student than traditional public schools, and receive no facilities funds. The number of charters providing a longer school day grew from 23 percent in 2009 to 48 percent in 2012. General structure and characteristics The rules and structure of charter schools depend on state authorizing legislation and differ from state to state. A charter school is authorized to function once it has received a charter, a statutorily defined performance contract detailing the school's mission, program, goals, students served, methods of assessment, and ways to measure success. The length of time for which charters are granted varies,