For most students, half the fun of a school field trip is the bus ride. But recently, when pupils at Olde Creek Elementary School in Fairfax County were to visit the Natural History Museum on the Mall, Virginia Williams refused to let her 5-year-old daughter ride in the yellow, 66-seat school bus.

Instead, she drove Elizabeth herself. "It was the fear of having my child on the open highway without a seat belt," Williams said.

Jenny Boyle, 6, said she experiences that fear every day as she rides the school bus from Braddock Elementary School to her home in Annandale. "Without seat belts, I'm scared," said the first-grader. "I sit back against the seat so I won't bounce up and down."

A growing number of parents, citing the deaths of 17 children and injuries to 3,300 last year in bus accidents, have joined the National Coalition for Seat Belts on School Buses.

At least 15 school districts nationally have mandated the belts, and the Virginia and Maryland legislatures are expected to be asked next month to require that school buses in those states have them. The District, which provides bus service only for its 2,000 handicapped students, has had seat belts on buses for 20 years, school officials said.

Dr. John States, a professor of orthopedics at the University of Rochester who has studied passenger safety in cars and buses, said seat belts on school buses would prevent many of the head and spinal injuries that come from "being thrown to the roof of a bus in a rollover or . . . to the opposite side of the bus if it's hit on the side."

But some federal officials argue that seat belts are not needed. "School buses are far and away the safest form of highway transportation in the country," said Richard Burdette, the National Highway Traffic Safety Administration's director of public and consumer affairs.

"I am aware that school buses are one of the safest places a child could be," said States, "but seat belts would make them even safer."

In addition to safety, the 11-month-old coalition is worried about the mixed signals children get when they must wear seat belts in cars but not on school buses.

"It's a product of the child restraint law," said Laura Schwartz, membership chairman of the coalition. She said that the movement for school bus seat belts began in the late 1970s, when states began passing laws requiring that children up to the age of 5 be strapped in when traveling in a car. The result, said Schwartz, is that today's parents are "outraged at the inconsistency" of having their children securely fastened in a car but unrestrained on their way to and from school.

"If I drove Jenny to school, I'd be breaking the law if I didn't buckle her up," said Betsy Boyle, Jenny's mother and the coalition's Virginia coordinator.

Joyce Bilodeau has patiently strapped her daughter Kristina in the family car every time time she takes her 5-year-old on a trip. Recently, said Bilodeau, Kristina has asked to remove her seat belt in the car.

Boyle, a Fairfax County mother, was one of the leaders of a movement that won a resolution last month from the Virginia Congress of Parents and Teachers asking the General Assembly to mandate seat belts on both existing and new school buses. A similar resolution is to come before the Montgomery Council of Parent-Teacher Associations Jan. 29.

Virginia Del. Dorothy S. McDiarmid (D-Fairfax), chairman of the House Education Committee, said, however, that money for new projects is unlikely to be approved at a time when "the temper seems to be against tax increases."

On the national level, Rep. Peter Kostmayer (D-Pa.) introduced a bill last April to reimburse school districts for half the cost of buying seat belts and to provide federal money for educating children about school bus safety. Three months ago, Rep. Lawrence Smith (D-Fla.) introduced a bill that would withhold federal education funds from school districts that do not pass a mandatory seat belt law for new buses.

The cost of installing seat belts on school buses -- a minimum of $1,000 in the standard $35,000, 66-passenger bus -- is not the main reason for the opposition of manufacturers and some officials at the National Highway Traffic Safety Administration, those officials say.

Federal officals argue that the high-backed, padded seats and stronger frames, required on post-1976 school buses, confine students to a small space and are protection enough. They say most of the fatalities tend to occur as children make their way to and from the bus, rather than on the bus itself.

In Fairfax County, C. Frank Dixon Jr., assistant director for school transportation, said, "Seat belts would have made no difference at all in any bus accidents in the past five years." In the past 2 1/2 years in Fairfax, there have been 81 accidents on school buses, including some resulting from roughhousing among children, and the worst injuries were broken collarbones, according to school officials.

Some opponents of the movement, however, say seat belts could expose children to more danger because they can be used to hurt others in child horseplay, can subject children to the "jackknife" effect if the bus crashes or stops suddenly, or can leave them dangling in their seats if it rolls over. Also, they say, straps can make it harder to evacuate a bus.

Advocates say the principle behind having seat belts in cars -- a lower risk of death or injury -- applies to buses as well. Also, Fast said, buses would be involved in fewer accidents because strapped-in children are less likely to distract the driver.

Even some officials at the traffic safety administration have altered their ideas on the seat-belt issue. "We continue to believe that seat belts would not measurably improve the safety of children on school buses," said Burdette. "But if seat belts on school buses were to help children use them in cars -- that's where we might see some payoff."

Frederick Spry, director of transportation for the Greenburgh, N.Y, school district, which in 1979 became the first in the nation to require seat belts on school buses, calls the belts "a necessity."

"It's another asset in the safety of our children," he said. "I mean, we're talking about human lives."