The FMVSS number for the school-bus roof-strength standard is 220.
Federal Motor Vehicle Safety Standard No. 220 is officially called School Bus Rollover Protection. Its purpose is to reduce the danger of a school-bus body collapsing during a rollover. The test applies a force to the roof equal to one and a half times the bus’s unloaded vehicle weight, with limits on how far the roof may move.
FMVSS 220 is often confused with two neighboring school-bus standards. FMVSS 221 concerns the strength of body joints, helping prevent panels from separating and exposing occupants. FMVSS 222 covers passenger seating and crash protection, including protection from impacts inside the bus.
Another possible mix-up is FMVSS 216, the general roof-crush-resistance standard for certain passenger vehicles and buses. School buses have their own dedicated rollover-protection rule, so the relevant number for this question is 220, not 216, 221, or 222.