Going over an overpass, seatbelts would not have done a bit of good.
Skrool buses are designed to take front and rear impacts. Nothing unusual, or any cars fro that matter (unless NASCAR, they have a roll bar cage system), if it should land on the roof, would crush. This one appers to have landed front/headon and landed upright.
If it landed on its roof, much worse.
I suppose most of the injuries are broken necks, spinal compressions, etc. that are fatal...or possibly blunt-force trauma.
Seatbelts DO NOT save all lives. I know this, unfortuneatly, for a fact. My 16 year old brother was killed on 21 July 1991. He had his seat belt on, his friend did too (he survived, state troopers said he survived because he broke the seat, the friend of my brother is obese). The car ended up inverted, trunk crushed, etc. Speed and overcorrection on a county road late at night, with no markings on the road, are the probably culprits (as well as inexperience, he had his license exactly 3 months, his BD was 21 April 1975).
As a note, a year later, TD ran a "story" where all the fatal crashes in Laud County in 1991 no one wore a seat belt. Lets just say when I read that, I called the newsroom, and someone lost some butt in that phone call. Next day, TD retracted the story.