After the summer field trip, Teshigawara and Mochizuki re-record Matsunaga's message about how to stop the Calamity after it's started - with an emphasis on the actions being discussed, so that no one regrets the decision, and also to prevent the same insanity that took over the entire class during the summer trip.
However, that happened after the Extra had been killed. It was revealed earlier, from Matsunaga's account of killing the Extra, that only the person responsible for the Extra's death will remember it for a short while. When Sakakibara showed everyone the class photos that were taken when they arrived, no one sees or even knows of the Extra which Sakakibara and Mei can see and remember.
Sakakibara remembers Reiko because he was the one who killed her after learning that she was the Extra. Mei remembers, because not only was she there and about to do the killing herself, but as she also revealed, she remembers seeing Reiko die. Her "false eye" could also contribute to her remembering.
However, since only Sakakibara and Mei remember the Extra, how do Teshigawara and Mochizuki remember that it was the death of the Extra that stopped the Calamity? since it was once that killing the Extra stopped the Calamity (so it wasn't proven that there was a link)