Summer school enrollment is up sharply throughout the Washington area this year, as school districts have embarked on an unprecedented effort to convert the traditional vacation break into a study period for struggling students.

Fairfax and Prince George's counties are forecasting enrollment increases of 20 percent, Alexandria expects a 15 percent rise, and Montgomery County officials say registration for summer school is running about 10 percent ahead of last year's total. The highest concentration of summer students is in the District, where 30,000 children -- 40 percent of all the students in the D.C. public school system -- will be taking remedial reading and math courses.

More children are trading the beach for the books because school officials were more aggressive this spring in recommending -- and sometimes requiring -- summer classes for students with low grades or lagging test scores. And although the remedial classes are usually optional, school administrators say most parents are accepting the advice and signing up their children, regardless of the disruption it might cause to the family vacation.

Traditionally, summer school has drawn a mix of weak students taking classes they failed during the regular school year and highly motivated students who want to finish graduation requirements early. This year's enrollment jump is due mostly to a third group, school officials say: students who didn't flunk a course but were still deemed in need of extra instruction.

In many cases, high-stakes testing programs are prompting school administrators to open summer classes to more children. Fairfax and Loudoun counties, for example, are urging children who did poorly on the Virginia Standards of Learning exams to return to campus in August for a two-week remedial session that will focus heavily on their test-taking abilities.

In the District, where summer enrollment will be 17 percent higher than last year, most of the students taking the classes are children who scored far below grade level on the Stanford 9 Achievement Tests given in April.

"There is more riding on student achievement, and we want to help them catch up with skills they might have missed during the year," said Barbara Hunter, a spokeswoman for Alexandria public schools, where enrollment in summer remedial classes may top 2,700, compared with last year's figure of 2,349. The number of students taking summer enrichment classes is expected to hold steady at about 350.

The enrollment surge highlights questions about summer school curriculum, which has long been considered by many educators to be inferior to that taught during the regular school year. School officials said they are beefing up lesson plans to make sure that the July and August sessions are worthwhile.

The classes typically are held Monday through Friday mornings, starting shortly after Independence Day and ending in mid-August. They are usually taught by regular teachers, and school officials say they have had no trouble finding teachers who want to earn the extra pay.

The same enrollment trend is occurring nationwide. Schools in New York City, Chicago and Florida, among many others, are cutting students' vacations short for intensive tutoring in basic skills. In some cases, students are being told that they must attend summer school if they want to move on to the next grade, part of a school crackdown on "social promotion," the practice of letting struggling students advance so they can stay with their age group.

Most of the Washington area students who will attend classes this summer aren't in danger of being held back.

"These are children who might be passing, but the schools and their teachers feel that they may benefit from some additional help," said Leroy Tompkins, chief administrator for instruction in Prince George's schools.

In a 1997 national survey by Public Agenda, a nonprofit research group in New York, 40 percent of students and 50 percent of teachers said students are not expected to work very hard in summer school, and 58 percent of teachers said the classes are easier academically than courses taught during the regular school year.

Washington area school administrators said they are well aware of summer school's reputation as a holding cell for failing students. But they said the quality of instruction can be high if teachers are following a well-designed curriculum. Filling out work sheets or reading silently for days on end won't hold a student's attention, they said.

"Historically, we've given teachers lots of latitude," said Terrence W. Hill, director of secondary education for Loudoun County schools. "Now we're trying to do some testing upfront, find out what the students are weak in, and develop some lesson plans around those weaknesses."

Hope Hummel, a veteran high school English teacher in Montgomery County, said that the summer curriculum is the same material she teaches during the regular school year, but that it is more concentrated because an entire year of instruction is squeezed into six weeks.

This summer, her ninth-grade English students will read four classics: "A Separate Peace," "Raisin in the Sun," "Of Mice and Men" and "Romeo and Juliet," as well as some short stories and poetry. They will practice grammar and writing and prepare for the Scholastic Assessment Test with drills in sentence completion, vocabulary and analogies. There are daily quizzes, three composition papers and a final exam.

"It's serious school and it's very intense," Hummel said. "I don't think summer school is in any way a slide-by."

In remedial classes for younger children, teachers say they use games and other "fun" techniques to excite students.

Julia Tansley, whose 7-year-old son, Justin, will attend summer classes to improve his reading ability, said her perception of summer school has changed.

"The school is really trying to establish a positive atmosphere," said Tansley, of Ashburn. "In the letter the school sent home, they said it was a `privilege' for students to go to summer school. My son is looking forward to it. He doesn't feel like he's being punished."

Vanessa Pakla's 9-year-old son, Andrew, also is going to summer school in Loudoun for four weeks. That means the family vacation to Upstate New York is off-limits until the middle of August. And as for her accounting job with the flexible hours, well, they're going to get even more flexible.

Nevertheless, Andrew's mother could not be more pleased.

"I think summer vacation is too long these days," said Pakla, also of Ashburn. "I want him to continue to work on his reading, and there's still enough time for play."