In A Certain Magical Index volume 15, ITEM mentions that they killed SCHOOL's sniper "three days ago", without any further elaboration. Rather than assume another fight with Yumiya a day or two after this one, it could be that this episode's fight is what they were referring to, erroneously believing she was killed. The exact date is unclear, but it's certainly within a few days of the Battle Royale incident on October 9th. And by the end of this episode, SCHOOL is already arranging to bring in Sunazara to replace Yumiya, so her death isn't necessary to account for that. In other words, Yumiya could still be alive and out there somewhere.
