If Inori could destroy the virus by singing why didn't she do it before? In episode 11 Keido releases the Apocalypse Virus, Inori stops the outbreak and cures all of the infected by singing. If she is able to stop the virus by singing, then why can't she save everyone who is already infected?
Inori could only delay the effects of the virus. Exactly how long and to what extent, it is unknown. She has that ability because


   she is the patient zero of the virus, and the vessel for Mana, and that gives her some control over the virus.


Also in this episode, she is not quite herself, because


   Mana's consciousness implanted on Inori is beginning to surface. That is why she kills the students attempting to undress her, and why she stabs Arisa.


But I think maybe you are mistaking ep 11 with the ending, because


   It is Shuu's void power (absorb the problems of the world unto himself) that finally cleanses the world of the virus.


Alas at the very end she goes on and


   by symbolically accepting the "Guilty Crown", Shuu shares his fate with Inori, and then she removes the virus from him, along with the king power and the void powers (see note below). Inori then crystallizes and dies.


Footnote:


   Void powers were created by fragments of Mana inside the teenagers, once they were absorbed by Shuu, no one else had voids.