(Michael Conroy/AP Photo)

Two recent school bus accidents, one in Indiana and the other Washington State, have left one student and a bus driver dead, and scores of students injured, raising new concerns about school bus safety.

In Indianapolis, a bus carrying 50 students, ages 5 to 16, to the Lighthouse Charter School ran into a concrete bridge abutment. The 60-year-old driver of the bus was killed, as was 5-year old student Donasty Smith. Two other students were critically injured.

"A school bus crashed into a bridge, and the school bus driver's down," a caller to 911 said.

"OK, are there kids on the bus?" asked the 911 dispatcher.

"Yes," said the caller. "Just getting them off now."

The bus was badly mangled, and although some children were able to scramble off the bus with the help of others, four passengers had to be freed by the fire department.

According to ABC News sources it appears preliminarily that the school bus driver either was distracted or had a heart attack while behind the wheel. He appears to have hit the overpass without braking, sources said. Investigators will not be able to rule out a heart attack until an autopsy is performed.

In the other accident, in Quincy, Wash., the school bus rolled over, apparently after it veered off the road and the driver overcorrected. There were 38 students on board. One was critically injured and remains in the hospital. Three were seriously injured and have been treated and released. The bus driver also remains hospitalized.

The accident, about 120 miles east of Seattle, occurred on a rural bus route. The bus picks up students of all ages, from kindergarten to high school. Initial reports are that alcohol, drugs, and the weather, were not factors in the accident.

Neither of the buses was equipped with passenger seatbelts, which the National Highway Traffic Safety Administration does not require in larger school buses. NHTSA and school bus manufacturers say the buses are safe because of their size, and what's called "compartmentalization." Buses are designed to hold students in place with the help of narrow widths between seats and high seatbacks.

"We feel strongly that school buses continue to be the safest way to transport students," NHTSA spokesman Lynda Tran told ABC News. "They are even safer than their parents' cars."

The government points to statistics that underscore the safety of school buses, which transport 23 million children a day. According to the NHTSA, about 800 school-aged children are killed in motor vehicle accidents during normal school travel hours each year. Only about 20 of those deaths are school-bus related - an average of five school bus passengers and 15 pedestrians, often students hit inadvertently by the school bus, according to the NHTSA statistics.

But the American Academy of Pediatrics strongly disagrees that seat belts aren't necessary on scho0l buses. It wants all new buses equipped with lap/shoulder belts to "ensure the safest possible ride", according to Dr. Phyllis Agran, a pediatrician.

Agran said that according to her research, approximately 17,000 children are treated in emergency rooms annually, having been injured in school buses, with 42 percent of those injuries involving crashes.

School buses are "a dinosaur with respect to occupant protection," she said. Compartmentalization was a safety concept from the 1960s , before there were mandatory requirements for lap/shoulder belts in motor vehicles, she said. Seat belts in school buses "should be a no-brainer by the year 2012," she said.

The government strengthened its compartmentalization rule in 2009 to require higher seatbacks in new buses, for even greater protection. As for whether larger buses are equipped with seat belts, that decision has been left up to the states and individual school districts.