Do the feet you rolled before you hop on the bus get away? Like if 2 feet are in play and i say i want to get on the bus, do i roll those 2 feet with the bus?
The School Bus die only replaces the drawing of one die from the cup, so since you rolled two feet dice, and you need to draw one die from the cup, the bus would replace that one die drawn. You would still roll the other two dice, since you "always roll the School Bus die in addition to two regular dice" as per the rules of the expansion.
Hopefully that helps!