WASHINGTON (By Ben Feller, AP) June 3, 2007 — More than 30 years after becoming the largest group of schoolchildren in U.S. history, baby boomers have finally lost their record — to their kids.

A total of 49.6 million children attended public and private school in 2003, beating the previous high mark of 48.7 million, set in 1970 when the baby boom generation was in school.

The growth is largely due to all the children who were born in the late 1940s to early 1960s and have since become parents themselves, the Census Bureau said Wednesday.

Rising immigration played a part, too, in pushing enrollment past the 1970 record of 48.7 million.

''You could have predicted this back in 1970 when we had all those kids,'' said Mark Mather, a demographer for the Population Reference Bureau, which assesses population trends.

Even if it isn't surprising, the record tally of students in the first 12 grades poses steep challenges for schools: recruiting teachers, helping children who don't speak English, keeping class sizes manageable and coming up with enough financial aid for college students.

In population rings outside urban areas and in Western states such as Nevada and California, as examples, the growth has been concentrated, increasing demands on schools.

''They just really don't have the fiscal capacity to match this,'' said Scott Young, senior policy specialist in education for the National Conference of State Legislatures.

In districts outside Atlanta, Houston and Las Vegas, enrollment has soared more than 20 percent in last five years, said Bruce Hunter, who directs lobbying for the American Association of School Administrators.

His group has identified more than 400 such districts.

Immigration helped fueled the boom.

A total of 22 percent of students had at least one foreign-born parent, including 91 percent of Asian children and 66 percent of Hispanic youngsters.

Many high-growth states have not prepared well for that racial and ethnic transformation, said Patrick Callan, president of the National Center for Public Policy and Higher Education.

With the largest-ever high school graduating class coming soon, colleges are being pressed to provide capacity for everyone while keeping tuition affordable.

''These kids are coming along at a time when — unlike the baby boomers — their chances of a middle-class life without college are almost nil,'' Callan said. "It's going to drive higher education policy over the next few years. This is a huge challenge.''