On the one hand, I'm inclined to think there should be seat belts for kids over a certain age. They do serve a purpose, for the most part. Then again, I've been three serious car accidents. One was when I was ten, and someone hit my father's car (full of family, including a baby), and the car spun around and came close to landing on its roof before miraculously bouncing back up. (It was big, old, Buick.) Nobody, including me (and I was a very small 10), my sister, my five-year-old brother, the baby cousin, and the three adults, was hurt.
Second car accident: I was sixteen. No seatbelt. Broken two legs and got a concussion but wasn't more seriously hurt. Third accident: I was twenty. I had no seatbelt on. The driver, my girlfriend, faithfully wore a seatbelt; and I suspect she may have died because of it. She was a rail-thin, small, young woman; and it was said that "something in her spinal cord snapped". I, on the other hand, did go into the windshield and end up with all kinds of injuries - but obviously, I lived.
Twelve-year-old kids are still often pretty small. There's a reason children under a certain age/weight must ride in car seats, and that reason is that the seat-belt can hold them in around their hip area, while shoulder straps (not always adequate, functioning/fastened properly) don't always hold the upper body in place. As a result, injury can be worse than it might have otherwise been. That's not true all the time, of course; but it's enough of a concern to make the seatbelt issue not quite as clear-cut as many people seem to think it is.
So, while I lean toward thinking seat-belts in buses are a sensible and important thing, I can't say I'm 100% sure about even my own leanings. With the accident that killed my 100-or-so-lb friend, I, too, was a 100-or-so-lb person. I'm not sure she wouldn't have been better off being knocked out of her seat.