(NBC News) Most experts will tell you the bus is the safest way for kids to get to school.
25 million children ride every day, and on average only four die each year.
Safety experts say lives are saved by a bus design called compartmentalization.
"Closely placed, energy absorbing seats. And those energy absorbing seats are designed for front crashes," explains the National Transportation Safety Board's Kristin Pond.
Buses also absorb much of the impact when hit from behind, but a rollover accident can have a very different outcome.
"If the bus is hit from the side or it rolls over compartmentalization means nothing," says Janette Fennel, founder of KidsAndCars.org. "Now if you have them in a seat belt, they will stay restrained and injuries will be minimum."
Only six states require seat belts on school buses. There's no federal mandate, but that could change.
"This fall, we're going to come out with some action, and that could be inclusive of seat belts, very likely to be at the top of the list," says National Highway Transportation Safety Administrator Mark Rosekind. "It's not just that they're in the bus. They've got to be worn properly, right, and we've got to make sure that it's not the two, but the three-point."
And what happens if there's an accident and kids are trapped in their seat belts and can't escape? These are things federal officials are looking at,and trying to work out before making their announcement this fall.