The Extra Student Can Leave the City
>!There is no beach in Yomiyama, at least according to the light novel. I honestly can't remember whether this was changed in the anime, since there is an episode set at the beach. So, based on that alone, we could already conclude that the Extra Student of that year can leave the city, since Sakakibara's aunt was there with them.
But going back to the books, this is confirmed in Another Episode S. Teruya Sakaki spent three months outside the city at his father's mansion with his friends, who were afraid of the curse. He took a picture with them, and in that photo, there is an unnatural gap, as if someone were missing. And someone really was missing: Satsuki Shinomiya. This is revealed at the end of the book, when Mei puts the pieces together. Satsuki was the calamity that year, which is why she disappeared from the photograph.
So, this means that the Extra Student can officially leave the city, even though the curse only affects people in that town. What would happen if someone who had studied with Satsuki in the year she died had left the city and encountered her outside Yomiyama in 1987, after she returned as the calamity?
We know that the curse only affects people in that town. Sakakibara's father knew that Sakakibara had been to Yomiyama, even though the curse wouldn't let him share that information. Whenever they spoke on the phone, there was always static.!<