The U.S. Education Department has announced an income and assets test for federally guaranteed student loans that will allow some families earning more than $100,000 a year to qualify for the 9 percent loan money regardless of the amount of assets they have in stock, real estate or other investments.

The so-called needs test was required by Congress as part of its budget reconciliation bill in late July for all loans to families earning more than $30,000 a year. But the administration proposal, published in the Federal Register last week, sets the standard for need so generously that student aid experts say it will have little impact in limiting the heavily subsidized program. This year, the program exploded to a projected $7.8 billion to $8 billion in loans to more than 3.6 million borrowers.

Until Oct. 1, when the new limits are scheduled to go into effect, the loans are available to all college students with no limit on family income or assets.

Because of widespread publicity about possible limits on loan eligibility, there has been an unprecedented rush by students to get their loans before Oct. 1. As a result, College Board officials estimated that only about 10 percent of all students enrolled this year might be affected by the new income limits.

Because of the upsurge in lending and the high interest subsidy the government pays, the Reagan administration last winter proposed limiting all loans to the amount students need based on a formula involving college costs and family income and assets.

The administration said it had in mind a needs test devised principally by the College Scholarship Service of the College Board, one that most colleges themselves use in distributing financial aid.

The limits were opposed by most higher education groups and congressional Democrats who had opened the program to all students in 1978.

In the House-Senate conference on the budget bill, the Democrats succeeded in getting the needs test to apply only to families with an adjusted gross income over $30,000 and included language directing the secretary of education to consider excluding the value of a family's main home in counting its assets.

David Bayer, a career official who is chief of the Education Department's guaranteed student loan branch, said last week that the actual needs test was hurriedly devised in mid-August by a committee of department officials, congressional staffers, and college financial aid experts.

"We wanted to keep it as simple and straightforward as possible," Bayer said, "and get something in place in a hurry."

He stressed that the new formula would be in effect only during the academic year that ends next June 30, and said he expects the Reagan administration to propose major changes to limit eligibility and cut costs after that.

Under the new formula, most of the students excluded from the loan program will be those from families in the $30,000 to $45,000 income range -- just above the $30,000 cutoff -- who attend relatively low-cost public colleges. Public college costs average $3,873 this year.

As college costs increase, which they do at most private colleges, the income limits go up considerably. For those attending average-cost private schools -- $6,885 this year -- loans could be made to families with an income of up to $46,500 if one child is in college and up to $79,250 if two children attend college.

For high-cost private schools, such as those in the Ivy League where expenses commonly top $11,000, loans are available for families earning up to about $110,000 with one child in college and for those with two children in school the earnings limit would go up to $130,000.

The new loan eligibility limits can be vetoed by either house of Congress before Oct. 1, but an aide to Rep. Peter Peyser (D-N.Y.), who took the lead in pushing for generous loan limits, said Peyser was satisfied with the administration formula.