WASHINGTON — It would be hard to find a student at Stone Bridge High School who has never used the Internet for a research assignment, socialized with Facebook or played a video game.

But few know much about how computers and the Web actually work.

About 70 students at the school in suburban Loudoun County, Va., are taking introductory or advanced placement courses this year in computer science, getting a glimpse behind the games and Web sites they use all the time.

For 1 1/2 hours, they enter a bracket-and-parentheses-laced world and practice speaking and writing in Java and C++. If they stick with it, they will be part of an elite group that is exceedingly employable, economists say, even in a recession.

In suburban Loudoun County, sometimes called the Silicon Valley of the East, fewer than 5 percent of all high school students took a computer science class in each of the past three years, and the numbers are slipping slightly.

Nationally, the portion of schools that offer an introductory computer science course has dropped from 78 percent in 2005 to 65 percent this year, and the corresponding decline in advanced placement courses went from 40 percent to 27 percent, according to a survey by the Computer Science Teachers Association.

In the spring, the College Board, citing declining enrollment, canceled its AP computer science AB class, the more rigorous of its two courses in the subject.

The result of sporadic or skimpy computer science training is that a generation of teenagers great at using computers will be unlikely to play a role in the way computer technology shapes lives in the future, said Chris Stephenson, executive director of the Computer Science Teachers Association.

"Their knowledge of technology is very broad but very shallow," she said.

That has economic implications. "If you look at history, the nations with economic superiority are building the tools the rest of the world is using," Stephenson said.