The poor performance has pushed public and private efforts such as the No Child Left Behind Act. In addition, the ratio of college-educated adults entering the workforce to general population (33%) is slightly below the mean of other developed countries (35%) and rate of participation of the labor force in continuing education is high. A 2000s (decade) study by Jon Miller of Michigan State University concluded that "A slightly higher proportion of American adults qualify as scientifically literate than European or Japanese adults". In 2006, there were roughly 600,000 homeless students in the United States, but after the Great Recession this number more than doubled to approximately 1.36 million. The Institute for Child Poverty and Homelessness keeps track of state by state levels of child homelessness. The test scores of students attending U.S. public schools are lower than student scores in schools of other developed countries, in the areas of reading, math, and science. Out of 21 industrialized countries, U.S. 12th graders ranked 19th in math, 16th in science, and last in advanced physics. Formal education in the U.S. is divided into a number of distinct educational stages. Most children enter the public education system around ages five or six. Children are assigned into year groups known as grades. The American school year traditionally begins at the end of August or early in September, after a traditional summer vacation or break. Children customarily advance together from one grade to the next as a single cohort or "class" upon reaching the end of each school year in late May or early June. Depending upon their circumstances, children may begin school in pre-kindergarten, kindergarten or first grade. Students normally attend 12 grades of study over 12 calendar years of primary/elementary and secondary education before graduating and earning a diploma that makes them eligible for admission to higher education. Education is mandatory until age 16 (18 in some states). In the U.S., ordinal numbers (e.g., first grade) are used for identifying grades. Typical ages and grade groupings in contemporary, public and private schools may be found through the U.S. Department of Education. Generally there are three stages: elementary school (K–5th/6th grade), middle school (6th/7th–8th grades) and high school (9th–12th grades). There is considerable variability in the exact arrangement of grades, as the following table indicates. In K–12 education, sometimes students who receive failing grades are held back a year and repeat coursework in the hope of earning satisfactory scores on the second try.