Sean Murphy needed only one day to decide whether he wanted to start college before he graduated from high school. Murphy, a Robinson High School senior, learned from a neighbor last month that if he was not enrolled in college by may 1, he would lose his monthly Socila Security check.

Two days later, Murphy attended his first class as a freshman at George Mason University.

"It was quite a shock," said 18-year-old Murphy, now enrolled simultaneously at Robinson and George Mason. "And the whole thing was very confusing. I can see the need for making budget cuts, but what could be more important than education?"

Murphy and at least 500 other college-bound high school seniors in Northern Virginia have been caught in the current wave of budget-cutting economics, resulting in much scrambling and juggling for students, guidance counselors and local college officials.

As part of revamped Social Security regulations passed by Congress last summer, children who receive Social Security payments--those with a parent who is dead, retired or disabled--and who do not become full-time students at college or another post-secondary school by May 1 of this year will lose their benefit checks, which average $259 a month, at age 18. Before this new law, youths could continue receiving checks after age 18 if they were enrolled in college full time.

The law also provides that payments to all student recipients be eliminated during summer months, that cost-of-living increases be stopped and checks be cut by 25 percent each year until all student benefits are phased out in April 1985.

The immediate concern is the May 1 deadline for college enrollment.

"It would have been very hard for me to go college without that money," said Murphy, who has received the checks since his father died last year.

John Trollinger, Social Security Administration spokesman, said the intent of the new regulation was "that this year's senior class was to be the first class not to receive benefits under the student program." But some area students have found a loophole in the regulation and have opted for early college enrollment in order to keep the money coming.

Most second-semester seniors need credits in only two courses--English and government--to graduate, explained Don Lewis, director of guidance at Arlington's Yorktown High School. Most of the nine Yorktown students affected by the change have opted to continue taking those two classes at the high school, sandwiched around the 12 course credits required of full-time students at NVCC, he said. Although this might mean going to college evenings and weekends, it allows students to continue in high school sports and other extracurricular activities.

A student also could withdraw completely from high school and, with the principal's permission, substitute college credits in English and government to fulfill the graduation requirements, Lewis said.

Social Security offices have been flooded with calls on the situation even though leaflets and press releases advising students of the new laws have been out since last fall, Trollinger said. Students did not receive individual notices, he said, because department computers are not programmed to keep track of the exact age of each child beneficiary.

Many parents and educators, however, contend that Social Security should have provided more information about the change.

"I think they have been enormously inconsiderate in letting these people know where they stand in regard to the change in the regulations," said Bob Simon, director of guidance at Bishop Ireton High School in Alexandria. "A lot of families have been negatively affected and upset."

Geri Brickner, whose son Barry is a senior at Potomac High School in Prince William County, criticized Social Security for not warning families affected by the change. "I thought it was nasty," she said. "All of a sudden they put this on us. What if I hadn't read about it in the paper?"

Barry, who has been getting a monthly check since his father died in 1976, starts at Northern Virginia Community College March 31. "I don't like the fact that my child will be in high school and college at the same time, plus working part-time," said Brickner. "But it's rough with just the two of us, and with the mortgage and all the utilities and insurance and food costs."

In this area, students have turned primarily to George Mason University and Northern Virginia Community College. GMU enrolled eight high school seniors in January, according to Helen Ackerman, manager of news and information. Dozens more have been calling the university, she said, "but since we are on a semester system, we can no longer enroll them, and we are suggesting they go to a community college."

NVCC, which operates on a quarter system, expects to enroll as many as 300 high school seniors for spring classes starting March 31.

"We are not making any changes to our admissions policy on the enrollment of high school students," said Max Bassett, dean for student services at NVCC. The college always has had an early-admissions program for qualified high school seniors, he said.

Bassett, however, advises students to check with Social Security on their eligibility before changing plans. It is a "serious matter to drop out of high school," he said.

Some students, in fact, have decided not to leave high school early. They weighed the option of keeping Social Security checks coming or finishing high school in the normal fashion, and decided to let the benefits expire.

As one student said, "They are cheating us in a lot of ways because the senior year is the big time, especially second semester, and we will not be there and will miss a lot of excitement about the prom and graduation."

"I feel it's unfortunate that these students have been caught up in this," said Becky Gates, community liaison counselor at NVCC's Alexandria campus, one of the school's five campuses. "They have been planning on this aid, and then they found out they weren't going to be able to get it unless they disrupted their lives."

Gates said she has spent most of the past month working with local high schools to expedite applications of students in this situation.

Melissa Smalling, 17, a senior at T.C. Williams in Alexandria, said she wants to go to Mary Washington College, Randolph-Macon College or James Madison University in September, but for now she will attend both high school and NVCC.

"I figure that my Dad had worked for so many years and he died in 1977 thinking that I would have this towards my education," she said. "I had really depended on it. . . . I don't see how they can just cut it off after people have paid into it.

"I can see that the government needs to save money, but not like this."