"Itsumademo Kagayaki wo (いつまでも輝きを)" by Shirahamazaka High School Chorus Club [Wakana Sakai (Ayahi Takagaki), Konatsu Miyamoto (Asami Seto), Sawa Okita (Saori Hayami), Taichi Tanaka (Nobunaga Shimazaki), Wien (Natsuki Hanae)]
Short little special inserted after the performance in ep13 of tari tari and runs in parallel time until the ending. Probably worth watching if you like Tari tari.
Random personal rambling: I think i watched the tv series in 2013 and its quite an weird experience going back to this after 3 years. Makes me wonder why they bother making such a short after the anime's been done for quite a while. The short gets quite sentimental and reiterates the bittersweetness of graduation and parting ways, and it makes me wonder if I would've gotten sad had i watched it back then or perhaps that effect
works better after the fact. Because, I was in university back then, and now I don't keep in touch with the friends i had back then i anymore, so in a way the timing in which I watched this short with 2 different perspectives time fits more appropriately.