Well that escalated quickly.
I thought that maybe no one else would show up, or Sanada-san would find Kokoro on the way to school or something. Not, just, the other kids don’t exist 
At this point this is my best guess. Or maybe there’s some sort of force that makes it so they can’t meet outside of the castle until after the year is up. If you can make magic mirrors to transport people to a magical castle, I will also accept either of these realities.
Yeah I’ve also thought that but the media, plus the game system dates them pretty firmly into +/-3 years or so. So even if they are time shifted, you’d think that the teachers would be like “oh they graduated _ years ago” or something.
I hope that Masamune comes back soon.. but I also get why he wouldn’t want to. Although maybe he comes back eventually because he wants to find the key so that he can forget about them 
