I think another big clue for their being a time difference is the fact that the kids are supposed to be soldiers. It’s kind of hard to think that people would be even waging high-tech warfare given the post-apocalyptic landscape. If you take into context that war is one of the theories for the calamity then the man-eater problem is probably related to the kids being used as soldiers in it.
Another interesting thing is that an earlier chapter mentioned that the cause of this war was that humans had paranoia about robots watching them. There are a lot of strange things happening in the school related to tech that are suspicious. For example, Tokio got a question on her test tablet asking if she wanted to be free that erased itself as soon as she asked the teacher about it. Another one are the strange incidents with security cameras (not showing Tokio in the incubation chamber and the picture of Mimihime being sent without her knowledge). Finally, the medical robot says that Tokio is fine, even though there is a possibility that she’s pregnant. All of this makes me think that the school’s AI is sort of going rogue.