Officials of Washington's six relatively high-cost private universities said yesterday that the Reagan administration's proposed cuts in federal student aid may have a "devastating" impact on their schools by forcing middle-income students to drop out.

The universities already are planning to hike tuition by 15 to 19 percent next fall, the officials said, partly to off-set slight reductions in aid voted last fall by Congress.

If the Reagan administration proposals for more drastic cuts are approved, "there will really be a disaster" in the 1983-84 school year, said Kenneth Kohl, financial aid director at Georgetown University and former head of federal student aid programs under the Carter administration.

"The upper income will be able to afford it, and I think we'll all continue to make sure that the low-income students can attend," Kohl continued. "It's the middle-income that will have to drop out."

He said some students would switch to low-tuition state colleges while others would simply quit school.

"I think about 10 percent of our students might have to leave," said Elizabeth Fonseca, financial aid director of Mount Vernon College. "That's 50 students but for a small private college with an enrollment of 500 that would be devastating. We might have to fold."

Kohl and Fonseca spoke at a press conference at Georgetown arranged by the Consortium of Universities of Washington. The meeting also was attended by officials and students from American, Catholic, and George Washington Universities and Trinity College. All of them predicted that the aid cuts would cause major hardships.

Three other consortium members--Howard, the University of the District of Columbia, and Gallaudet College--did not attend, though the Rev. John Whalen, executive director of the consortium, said they endorsed his statement denouncing the proposed cuts. The three schools charge relatively low tuition, receive large subsidies from the federal or city governments, and so far have been exempt from spending cut-back proposals.

Under the Reagan proposals, direct federal grants and loans to students would be cut about $1.5 billion in the 1983-84 school year below the $3.3 billion appropriated by Congress for 1982-83.

Eligibility would be tightened and costs to many borrowers sharply raised for the much larger guaranteed student loan program, under which $7.7 billion was loaned last year.

Since the Middle Income Student Assistance Act of 1978, direct federal aid has risen by about 60 percent while the guaranteed loans, which carry heavy federal subsidies, have more than tripled.

President Reagan said the cutbacks are part of a general effort to reduce federal spending and that the reductions will not affect "students with true need."

However, Fonseca said middle-class families with incomes up to $45,000 face serious financial problems when total costs average $8,000 to $11,000 for a year of college, as they will at the six private D.C. schools next year. Kohl said that by reducing the choices for middle-income students the cutbacks "will destroy the pluralism in higher education as it is known today."

The officials said their planned tuition increases are well above general inflation, which ran at about 9 percent last year, partly to provide scholarship funds for needy students.