The School Bus would have been the 7th vehicle in FoT, however, there were problems in implementing it. Section8 basically said that it looked like arse when it turned because it was so long, which is why it was never implemented.
The 6th vehicle, incidentally, was to be a taxicab. A yellow one, strongly reminiscent of the Checker Cab in Escape from New York. There are vestiges of this vehicle still in the game, but the sprite for it wasn't included. This sprite is one of several Holy Grails pursued by the FoT Mapping/Modding community. However, it looks like the closest we'll get to it is looking at the counter included with the Fallout: Warfare tabletop mass combat rules.
Cheers,
OTB