More than 60 percent of graduates from Loudoun County high schools earned an advanced studies diploma this year, the highest-level degree conferred by the state.

According to data released Tuesday by the Virginia Department of Education, 1,476 Loudoun seniors received the top diploma, which required them to pass 24 specific classes and nine high school Standards of Learning exams. An additional 881 students received the standard diploma, and 68 earned either modified standard or special diplomas, two categories reserved for those with disabilities.

Last year, 1,222 seniors, or about 59 percent of graduates, received the top diploma, state data show.

K. Anne Lewis, the director of guidance for Loudoun County public schools, said the advanced studies diploma comes naturally for students who have been enrolling in more challenging courses. "It's what colleges expect, and these are, for the most part, college-bound students," she said.

This year was the second in which students in Virginia were required to pass SOL exams to graduate. They must pass 22 courses and six SOL tests to receive the standard diploma.

Many critics of the SOL tests had predicted that graduation rates would decline under the new rules, but the rates have remained steady statewide. In Loudoun, 97 percent of high school seniors graduated, and the graduating class of 2005 was only 6 percent smaller than the class of students that began high school four years earlier.

Researchers often look to the latter number to determine how many students have dropped out over the four years of high school. Across Virginia this year, the graduating class was about 26 percent smaller than the ninth-grade class of four years ago. That statistic may be less meaningful in Loudoun, however, where rapid growth means students who drop out may be replaced quickly with newcomers.

Lewis said "very few" students had not graduated solely because of SOL exams. "We have done lots of work upfront to make sure that happened," she said.

Efforts included identifying students struggling with the tests early in their high school careers and providing them with extra tutoring.

Most seniors have already passed the exams they need for graduation, Lewis said. Encouraging students to stay focused on their studies in their final year and pass all their courses is often the harder task.