It could go either way.
If iOS 9 is a performance update, I can't see why not.
The iPad 2 still holds a fairly significant share of the iPad market. More people are on the iPad 2, than any other iPad, which says something. Depending on which statistics you look at it holds around 23-27 percent of the iPad Market share. The iPad Mini 1, which is still for sale is identical to the iPad 2 hardware wise (Besides bluetooth) and it holds around 20 percent of the market share. Apple don't have a habit of dropping support for devices they still sell till a year after their discontinuation at minimum. So if the Mini 1 gets iOS 9, the iPad 2 should as well.
The iPad 3 which accounts for around 16 percent of iPads in use, would presumably get iOS 9 if the iPad 2 does.
If one A5 iPad gets iOS 9, they all will. The iPad 3 has more ram than the Mini 1 and iPad 2, but that would rip off owners of the iPad Mini, as it is a newer device than the iPad 3. The iPad 2 being supported would mean that the newer iPad 3 would need to be supported and the identical iPad Mini would have to be supported. If the iPad Mini is supported, than the identical iPad 2 has to be supported , and the iPad 3 who is newer would logically be supported.
However if iOS 9 performs significantly worse on the A5, its less likely that the iPad 2/3 will get an update. Apple seem to have three factors in supporting devices with updates. They are: The length of time since the device was last sold, how well the update will perform on the device and how many people are using that device.