You must come to a complete stop before reaching a school bus which has stopped for passengers with its red lights flashing. When can you proceed?

You must stop behind a school bus which has stopped for passengers with its red flashing lights on. You cannot proceed until the red lights are switched off, the bus is in motion, or the bus driver has waved you through. When you do proceed, drive with care and keep watch for pedestrians. [Yield Right-of-Way to School Buses, Chapter 4: Right-of-Way, Texas Driver's Handbook]