Veterans as well as reservists are on the higher education trail in increasing numbers, thanks to the new GI Bill that also is giving a lift to colleges facing slumps in enrollment, a study reported Friday.

The American Council on Education, an umbrella group for the nation's more than 3,000 colleges, said the reduced number of high school graduates caused by declining birthrates was already being felt on many campuses.The report, "Joining Forces: The Military's Impact on College," said by 1993 more than 400,000 veterans a year are expected to pay college expenses with GI Bill entitlements.

And between now and 1991, the number using GI Bill benefits will more than double, rising from 150,000 to more than 300,000 a year, the study said.

Under the Montgomery GI Bill, named for its sponsor, U.S. Rep. G.V. Montgomery, D-Miss., active duty personnel are entitled to education benefits of $300 per month for 36 months in exchange for three years of service and a pay reduction of $100 per month for 12 months.

Benefits for reservists are $140 per month for 36 months, contingent upon signing up for a six-year term of service. Reservists may start cashing in their GI college education chips while fulfilling their commitment to serve.

Most recently, more than three-quarters of eligible men and women in the service were signing up for benefits, according to the report which noted:

"Between the beginning of the program in July 1985, and March 1988, about 483,000 personnel, or 65 percent, of the 744,000 eligible, signed up for the program."

The report said the American Council on Education estimates of larger numbers using GI Bill benefits are based on projections by the Veterans Administration.