Now that you said, you can also see in pages 14 and 15 that the ghost has the beauty mark on her right side and the original teacher has it on the left side. While on the page 17, the zoomed face of the ghost shows that she has the beauty mark on the left side, the same side of Mero-sensei. That is a good detailOn page 17, when Mero-sensei's crying for help, the panel is zoomed in on the ghost's face. Neat detail you wouldn't get on a first read, but it's likely signifying the real one in the mirror was actually the one shouting. Kaya's also a little confused when reversed sensei gets up but I guess she has more important things to worry about.