yall are overthinking this, the author was pretty blunt about having improvised poles sticking out of people's heads, just after we're told that zombies can be killed by injuring their heads. eko-chan is handling the 1am shift as shown by the timestamp. all she can see are the entrance/exit points of the building, not the common rooms.
the situation might be exactly what it seems to be: they killed each other. the more interesting question is why. maybe pay attention to who has head wounds (e.g kitakami) and who is more ambiguous. the placements also look relevant: at least one person was a suicide, there are standoffs between pairs, and piles that suggest a particular order of death.
or. risa-chan was doing something very important, same as in the first chapter when she was wandering around the school with a bloodied bat: putting zombies out of their misery, because she knew how they felt.