Here's the NCES's recent report "Status and Trends in the Education of Racial and Ethnic Groups"
Here are the tables with data on bilingual learners (also referred to as English language learners).
Mary Ann Zehr | Ed Week
July 20, 2010
About 5 percent of all elementary and secondary students in the United States speak a language other than English at home and have difficulty speaking English, according to parent surveys, says a federal trend report.
"The Status and Trends in the Education of Racial and Ethnic Minorities" report provides the latest data describing English-language learners in this country. It says that in 2007, the most recent year for which data is available, 11 million school-age children spoke a language other than English at home. That equals 21 percent of all students. The report says that one in four of the students who spoke a native language other than English at home had difficulty with English, according to their parents.
Another way of stating that last statistic is that three out of four children who speak a language other than English at home speak English with ease. That indicates to me that commonly, the children of immigrants are learning English.
The categories for reporting English proficiency are "very well," "well," "not well," and "not at all." If parents reported that children spoke English less than "very well," they were counted as speaking English with difficulty.