As 15-year-old Tanya Penny hobbled out of the hospital emergency room on crutches, shaken and bruised after her school bus overturned yesterday morning, her distraught mother uttered a simple plea: "All school buses should have seat belts on them."

Harriet Penny was echoing the sentiment of many worried parents who believe seat belts would reduce the number of injuries and deaths resulting from school bus accidents.

While national parent organizations, consumer groups and elected officials have backed the campaign to install seat belts on school buses, bus manufacturers and some school administrators have opposed it, saying seat belts would not improve -- and could reduce -- school bus safety.

Prince George's County school officials and the Maryland Department of Education say that seat belts can hinder evacuation in case of fire and can exacerbate injuries by making students jackknife against the seat in front of them.

"We don't think seat belts are the answer to providing child safety," said Richard Alexander, chief of pupil transportation in the Maryland Education Department.

County school officials said they did not know whether seat belts could have prevented some of the injuries that occurred yesterday when a bus packed with 48 students overturned on the Baltimore-Washington Parkway en route to Fairmont Heights High School. Dozens of youngsters were bruised and cut, and two suffered more serious injuries.

However, Dr. Lawrence Blob, director of emergency medical programs at Prince George's General Hospital, said that although most of the injuries were minor, "There's a good chance that if they'd all been wearing seat belts, there would have been fewer injuries."

Federal regulations require school buses built since 1977 to "compartmentalize" students on the "egg carton" theory of reducing injury by separating students and keeping them from being thrown in an accident. That is done, Alexander said, by putting high-backed, padded seats close together.

The Prince George's County Board of Education voted last year not to endorse state legislation that would have mandated school bus seat belts. Those who favor seat belts, however, point out that new laws mandate automobile seat belt use and child safety seats.

In Maryland -- where officials say only one child has been killed in a school bus accident in 25 years -- only Montgomery County school buses have seat belts for nonhandicapped children. Fifty-one of Montgomery's new buses have belts. Fairfax County recently purchased 70 school buses equipped with seat belts.