It's fairly common for younger children to speak of themselves in the third person; unusual for high school students to do so. But what's relevant here is that it creates an ambiguity.
@Oeconomist the whole "old man" bit us referring to how old men (stereotypically) re somewhat perverted (at least in Japan). So by calling the nurse an old man, she's referring to how she's kinda a pervert.
I wonder what's going on between the teacher and the nurse...
Well that's a plot twist and mimi really can't keep secrets it seems, so she's a zombie then... I'm not the only one who reached this conclusion it seems, real original