Washington ? A record 49.6 million students filled U.S. schools in 2003, breaking a mark set by their baby boomer parents and giving educators a new generation of challenges.

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.

“We knew they were going to have kids of their own. We have this classic echo effect going on.”

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, the growth has been intense, 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.

“The pressures are to stay up with it, to hire, to get the classrooms staffed, to find quality principals,” Hunter said. “But the joy of it is you have this tremendous opportunity, because the communities have a real clear stake, so you have vibrant school systems.”

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.

The enrollment growth is likely to continue through this year, according to the Census Bureau report. Enrollment is expected to drop slightly through 2010 – due to a decline in births from 1991 to 1997 – but then pick up again, the Census figures show.