According to the official website, the blu-ray of Amagami SS
volume 13 will include an unaired episode Tachibana Miya
arc. It will be released on April 20th 2011.
The episode number of Amagami series had been unknown due to the presence of the extra 13th volume. It's confirmed that the extra episode Kamizaki Risa
's arc will be aired on TV as the episode 25.
Source: Amagami official website