Also in episode 1 when the zombie was crawling over the girl and paused to snarl at her.
However, it isn't unreasonable to assume that there could have been some surviving zombies.
While the season wraps up relatively well, with the students finally settling at the quarantine camps, it also leaves some questions unanswered.
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She, along with the rest of the surviving group, trek out to the school.
The implication here is that Cheong-san is dead, which is most likely the case.