It turns out the 172x statistic is based on totals. The normalized fatality numbers based on vehicle miles traveled show that school buses are about 8 times safer than traveling to school in passenger cars.
No federal laws mandate seatbelts in full sized school buses, though NHTSA does have voluntary guidelines on the topic. Smaller school buses and any buses for Head Start programs may be required to have seatbelts and/or LATCH, depending on the weight of the bus and any relevant state or local laws. Seatbelt systems are available/optional on all new buses, but in many areas the school districts are not mandated to include them when they order new buses.
If your child is in a booster (usually 5+ years old and above 40 pounds), then they are probably safest without using a seatbelt or restraint system in a full sized school bus. Preschoolers probably do need a restraint system of some time, especially in a smaller bus.