The premise is that students take a special class before graduation, involving a shared dream that has some specific rules, but also strong elements of dream logic. The students need to deal with the various issues while competing for the key that will allow them to pass the class and graduate.
Those who fail simply take the class again the following week until they pass. This does mean that if I used the premise without modfication, I'd have to figure out what players with PCs who have passed do until the rest of the players' PCs pass. Passing takes the PC out of the game.
The characters in the manga learn or decide what is most important to them, making ethical decisions or trying to avoid them.
I think my favorite moment thus far is in volume 4, where a supporting character is about to graduate. The protagonist asks if he wants to talk to the girl he's in love with first, as he will never see her after graduation. The boy replies, essentially, "I made a promise not to come near her. And, even though this is a dream, and I am a cell phone in this dream, I am going to keep that promise." I like the mixture of honor and surrealism.