A 12.5 foot ladder will be a bit shaky if you need to get on the roof. My bus is almost that tall so the ladder would have to be straight up... not good. Look for something around 14 to 16 feet extended. That way the base of the ladder can be pulled away from the bus, reducing the chance that it will fall over backwards.

I bought this one a while back. Works well: 14.5 foot