The gate arms begin to lower while the school bus is already in the process of crossing the tracks. What should the driver do?

AAMVA CDL Manual Section 10.4.3, p. 10-9; 49 CFR Part 380, Appendix D, Unit D1.6 (Railroad-Highway Grade Crossings)

This is the opposite of what a driver does before starting across, and the two situations are easy to confuse under stress. Once the bus has committed to the crossing, finishing it quickly is safer than stopping or reversing on the tracks, even at the cost of a damaged gate arm.