Since school buses weigh so much more than most of the things they would collide with, they will generally emerge with less damage and fewer injuries due to a slower deceleration. A school bus colliding with a car is almost like a car colliding with a bicycle... you know which one will take the greater amount of damage. And as has been said, the kids would most likely not wear the seat belts anyway, plus, it would be very expensive to upgrade.
Though I definitely sympathize with anyone, or anyone who knows anyone who has been injured while in a school bus without seatbelts where seatbelts would have made a difference, I don't think the problem is that common. I hear about school bus fatalities very rarely.
Oh, and where I live, most public transit buses don't have seat belts either. It's not like they're targeting kids or anything.