Well, depends on what "floor" they're on. If its the floor of your bus, no, that's not a particularly good place.
Think about it this way - batteries contain sulfuric acid and they can emit hydrogen gas under certain circumstances. Should a battery explode (and I've had one explode), where is that sulfuric acid gonna land? Also, if the battery vents hydrogen gas, will that gas be at a sufficient strength to ignite and will it find an ignition source (open flame, spark, etc.)?
A proper battery box will contain the sulfuric acid in a leak/explosion, and it can vent the hydrogen gas to the outside atmosphere in a safe way.
(Note that a battery isn't a bomb. If it explodes its not going off like a hand grenade, throwing shrapnel everywhere - its usually just enough of a "pop" to split the case open and allow the contents to leak with a bit of splatter.)
Usually a battery box is made of a plastic or fiberglass that is resistant or impervious to acid. It usually has two vents - an intake and an exhaust. You put the fan on the intake so that the box is at a slightly higher pressure than "atmosphere" so that if the batteries vent, the hydrogen gas goes out the exhaust and doesn't pass the fan. If the hydrogen gas passes a DC fan, and that fan is of the "brushed" type, then that fan produces sparks by its nature and can set off the hydrogen gas. This also goes for the sulfuric acid because the acid can also atomize and you don't want it going past the fan.
And all of this isn't even touching the idea that hydrogen gas and sulfuric gas is not something you want to be breathing....