MIAMI -- Duna Lopez started school in Miami last fall not knowing a single word of English. After seven months, Duna, 8, is one of the most active participants in class.Her success is exceptional, but the language challenge she faced is increasingly common across the U.S. Nationwide, nonwhites are expected to become a majority of the population within a generation, and schools are at the cutting edge of that shift.School-age children who speak a language other than English at home are one of the fastest-growing populations. Their numbers doubled between 1980 and 2009, and they now make up 21 percent of school-age kids.In the 2009-10 school year, 4.7 million students were classified as "English language learners," or not proficient in English -- about 10 percent of children enrolled, according to the most recent figures available from the Education Department."This is part of a new reality that our public schools are facing," said Robert Linquanti, an expert in English-learner students for WestEd, an education research agency.Of all the challenges facing minority students and their schools, English learners are arguably the most disadvantaged. It's hard to find enough teachers who are qualified to instruct them, and there's little consistency in the programs used to educate them.Bilingual programs are gathering steam but provoking a sometimes heated debate with those who favor an English-only approach. English-learner students are more likely to be in poor, overcrowded schools and in many places represent an added cost to cash-strapped districts.The longer these students stay in special language programs, the further they fall behind in other subjects. In several states, their graduation rates are at less than 60 percent, including as low as 29 percent in Nevada, according to federal data.Seven percent of fourth-grade and 3 percent of eighth-grade English learners scored "proficient" or above in a nationwide reading exam, and thousands languish for years in ineffective English-as-a-second-language programs.The vast majority of English learners were born in the United States, and most are Hispanic. Overall, 38 percent of Hispanic fourth-grade students were identified as English learners, as well as 20 percent of Hispanic eighth-grade students, according to the 2011 National Assessment of Educational Progress math test.Latino students overall have some of the highest dropout rates and the lowest share of the population with a bachelor's degree. The language barrier does not affect the majority, but for those who enter school as English learners, the challenges are even greater.States such as Texas, California, New Mexico and Nevada have some of the largest proportions of English learners in their school-age populations. The students are also widely concentrated in low-income, urban schools. A study by the Urban Institute found that 70 percent are educated in 5,000 elementary schools -- 10 percent of U.S. schools.