School buses should not have seat belts, as it would increase the difficulty of children being able to get out in an emergency like a fire or flooding with no significsnt safety benefit.

Large buses are designed to not have belts intentionally. In fact, the designs in the US were developed with the National Highway Safety Transportstion Administration (NTHSA) and comparable agencies in other countries.

https://dmv.vermont.gov/faq/why-do-school-buses-not-require-seat-belts

NHTSA decided the best way to provide crash protection to passengers of large school buses is through a concept called “compartmentalization.” This requires that the interior of large buses protect children without them needing to buckle up. Through compartmentalization, children are protected from crashes by strong, closely-spaced seats that have energy-absorbing seat backs.

Small school buses (with a gross vehicle weight rating of 10,000 pounds or less) must be equipped with lap and/or lap/shoulder belts at all designated seating positions. Since the sizes and weights of small school buses are closer to those of passenger cars and trucks, seat belts in those vehicles are necessary to provide occupant protection.”