For Jim Whalen college began suddenly last week even before he graduated from high school.

Last Monday Whalen, 17, was a senior at Einstein High School in Kensington. The next morning, he was a freshman at Montgomery College, enrolled in four courses. Whalen had been admitted to the college the night before although he was failing high school algebra and still had not taken his senior mid-term exams.

"I'm shocked, really shocked," Whalen said about the abrupt transition, but the reason for the quick change, he said, was quite simple: to keep his Social Security benefits.

As part of the Reagan budget cuts passed by Congress last summer, students with a parent who is dead, retired, or disabled will lose their monthly benefit checks at age 18 unless they are attending college full-time before May 1.

Last year the benefits averaged $259 a month or about $3,100 a year. Nationwide, according to the Social Security Administration, about $2.1 billion was paid to 760,000 college students.

To beat the May 1 deadline, about 100 high school seniors have enrolled for the spring semester at Montgomery College. They will receive high school credit for passing their college courses, and should be able to get high school diplomas in June.

Officials said about 10 students have enrolled under similar arrangements at Prince George's Community College, about 25 at Anne Arundel Community College, four at the University of Maryland, and one each at George Mason University and the University of the District of Columbia.

Colleges in many states, most of them two-year community colleges, are making arrangements for high school seniors to beat the deadline, according to the American Association of State Colleges and Universities. The total number involved is not known, however, and some officials have expressed concern over how prepared some of the early admission students are for the abrupt change in their lives.

The student who made the switch to UDC, which has an open admissions policy, came from St. Albans, a highly selective private school in Northwest Washington.

James R. Brown, 17, who has been attending St. Albans on scholarship, said he receives about $180 a month from Social Security. "I was counting on that money to go to college," he said. Brown said he found out just two weeks ago that he would lose the Social Security benefit unless he got into a college quickly.

After one day at UDC, Brown said he thought its classes were "interesting." But he said he hopes to go next fall to Williams College in Massachusetts if he is accepted there with a large enough scholarship.

Across the country, about 150,000 high school seniors receiving Social Security benefits will be affected this spring by the cutback, according to John Trollinger, a spokesman for the Social Security Administration.

Trollinger said students could continue to get benefits no matter how many different colleges they attend as long as they start taking a full load of courses somewhere before May 1.

However, under the cutback, voted with little congressional opposition last August, all Social Security benefits to college students will be phased out by April 1985 with monthly payments being cut 25 percent a year.

According to the Associated Press, more than 700 high school seniors have enrolled for the spring semester at two community colleges on Long Island, N.Y. Delta College, a two-year public college in Michigan, plans to start some late spring courses a week earlier than the scheduled May 3 opening to accommodate 150 high school students.

"So long as the law is met, they're going to get the benefits," Trollinger said. "We are concerned that some students may be going to college prematurely and may be leaving itearly, too, because they have trouble."

In the Washington area no colleges are changing their schedules to help Social Security recipients, though some have allowed students without high school diplomas to take more courses than usual. Montgomery College said it has admitted some--with the consent of high school officials--who normally could not attend.

Spring registration at almost all local colleges wound up last week. However, at Northern Virginia Community College, whose classes are on a quarter system rather than semesters, spring courses do not begin until March 31. Officials said a large number of seniors may try to get in then as word spreads of the Social Security deadline.

Many of the students who just enrolled said they found out about the benefit cutoff from high school counselors and newspaper articles during the last few weeks. The Social Security Administration has issued leaflets, press releases and radio announcements, explaining the cutoff in student benefits, but it has not sent notices directly to high school seniors except the regular mailing that goes to recipients of children's benefits five months before their 18th birthday.

None of the official information explains that students can beat the deadline by attending college early.

However, in early January Montgomery County's guidance director, Darryl D. Laramore, distributed a notice to all county high schools, saying that Montgomery College would cooperate in arranging early admission. Rep. Peter Peyser (D-N.Y.), a foe of the benefits cutoff, has sent a letter--postage-free with his Congressional frank--to about 6,400 high school guidance counselors around the country, urging them to help students enter college before May 1 even if they have not graduated from high school.

Officials at several local colleges said some high school students, usually those with high grades, regularly take college courses, though only in rare special cases are students allowed to enroll full-time without a high school diploma. For students trying to beat the Social Security deadline some of the rules have been waived.

For example, at Prince George's Community College high school students normally are limited to three courses or 9 credit hours. They now are being allowed to take one course more to meet the Social Security requirement for attending full time.

At Montgomery College the high school students enrolling usually must have an average of 2.75 (B-minus) out of a maximum of 4. But James Darr, admissions director at the Rockville campus, said about half the Social Security recipients have been admitted with a lower average than that--sometimes a low C or high D.

"I have great trepidation about admitting students with very low GPAs grade point averages ," Darr said. "In a few cases I've counseled them strongly not to come. There's no point in dropping a student into a situation where you can forecast academic doom . . . .But with all this last-minute scurrying around to get Social Security benefits, folks look at the financial exigencies, and they want to come. It's hard to find the right remedial sections for them."

At George Mason University, officials said early admission is given only to outstanding students and only one has come this spring in an effort to retain Social Security benefits.

Ginger Ackerman, who has a 3.8 average (A-minus) at Mount Vernon High School, said she applied to George Mason in early January soon after she heard about the benefit cutoff.

"My dad died when I was 7, and I really do need the money $396 a month ," she said. "I wasn't planning on George Mason, but it's the closest university to my home . . . .I've applied to other colleges and I may transfer but I may not. I've moved into the dormitory. It's different here, but it's neat."

Social Security benefits were first given to college students in 1965. But as other federal student grant and loan programs expanded, several administrations, including President Carter's, tried to cut them out to bolster the financially strapped Social Security fund.

The critics charged that the benefits were not warranted because in contrast to most other student aid they are not tied to family income or college costs. Like other Social Security benefits they are a form of insurance with higher benefits--up to a maximum of $537 per month or $6,444 a year--going to the children of high-income workers.

In one case at Montgomery College the student enrolled is getting benefits because his mother died. His father, with whom he lives, is a GS-14, earning more than $40,000 a year as a federal administrator. At the University of Maryland three of the students enrolling early are seniors at Georgetown Prep, an expensive private school in Bethesda.

On the other hand, LaHugh Bankston, admissions director at the University of the District Columbia, said no students from D.C. public schools have applied for early admission even though their average income is low and a substantial number might qualify for Social Security. Bankston said he has received no request from D.C. school officials to help students take advantage of the Social Security benefits.

College groups that lobbied to keep the student benefits argued that in fact most of them go to low-income families, and that other aid programs cannot make up the difference when they are cut.

"Everybody would agree there are a lot of anomalies in the Social Security benefits ," said John P. Mallan, vice president of the American Association of State Colleges and Universities. "But Reagan is trying to cut a lot of other programs, too. There are many, many students who are going to be hurt."

John Sharon, a scholarship student at St. Albans who lives in Chevy Chase, said he decided not to rush into college this spring even though he will lose the Social Security benefit.

"Money's important but there are other things, too," Sharon said. "My hopes just aren't as high as they used to be."