African-American and Hispanic students — largely in urban schools — lag far behind white students, who mostly attend middle-class suburban schools. Critics argue that when Congress reauthorizes the 2002 No Child Left Behind Act (NCLB), it must retarget the legislation to help urban schools tackle tough problems, such as encouraging the best teachers to enter and remain in high-poverty schools, rather than focusing on tests and sanctions. Some advocates propose busing students across district lines to create more socioeconomically diverse student bodies. But conservative analysts argue that busing wastes students' time and that permitting charter schools to compete with public schools will drive improvement. Meanwhile, liberal analysts point out that successful charter programs are too costly for most schools to emulate, and that no one has yet figured out how to spread success beyond a handful of schools, public or private.