We have a local school bus that took a head-on collision at speed (bus traveling 45mph, oncoming pickup at 60mph) and treated the occupants very well. The bus was full of high schoolers, the pickup was driven by another high schooler. The bus driver took a break near the ankle and none of the students sustained anything more than bumps and bruises.
Now, a lot comes into play here - the bus outweighs the pickup by several times, so it has the momentum to absorb the oncoming momentum and keep moving. But it's certainly not like hitting a flying bug. All that energy from the truck headed the opposite direction has to go somewhere, and it slows the bus dramatically in an instant. And these are high schoolers - no matter how many times the driver reminds them to sit down, sit correctly, they just won't. And even after all that, with no one besides the driver in a seatbelt, the students were all OK. (To satisfy curiosity: the driver of the pickup took serious injuries of which none were life-threatening; he spent two days in the hospital. He'd gotten impatient driving behind a tractor/trailer and decided to pass immediately after a curve without checking the lane first. This was a two-lane state "highway" and he was indeed in a zone that allowed passing.)
Then there's the Really Bad Accidents where the bus ends up on its side or upside down. Seatbelts are designed the hold under stress-- the button becomes near-impossible to operate and release. Smaller children will require the assistance of an adult. And there's ONE on the bus. With 50 kids. They make a safe belt cutter that a driver can carry, but now the driver has to visit every seat with multiple belts, cutting kids free, supporting them so they don't fall. This is not a scenario I'd care to have enter reality.