Crazy Fucking Videos

Bigger problem with no belts is bus driving into wall. Or another bus. Or rolling. Or hitting a bump launch kids into the ceiling.

But yes, generally it's safer due to a few things related to their mass. Primarily, conservation of momentum and material strength/crumble physics. A truck or car in motion suddenly transferring that energy to a bus is going to have that energy distributed over a much larger mass, meaning the bus moves much less quickly. It also has more material to crumble and absorb the impact, making the collision more inelastic, distributing kinetic energy into deforming the metal instead of pushing the bus and occupants forward. Accordingly, it doesn't increase acceleration as quickly as you might expect which is what causes injury (the rate of change in acceleration is called 'jerk', btw, which is a pretty accurate name in collisions).

The truck driver also benefits from the crumble of both vehicles reducing the kinetic energy, a bit. But his vehicle has a lot less material to crumble before that material includes his organs. That's also assuming the vehicle isn't short enough to have the front end slip under the bus's bumper leaving their face to absorb the impact.