Let me see if I can talk about this without babbling [ETA: nope.]: Mashiro has grown up as a boy, but actually he's half girl (specifically, the lower half of his body is that of a girl). After noticing that students around him are disappearing, he's called to a special nurse's office and begins participating in a mysterious "class" that takes place in a dream world. The students all appear as they really are inside their heart (apparently in our hearts we are things like suits of armor and faceless girls and hands), so he doesn't recognize any of his classmates. They are all supposed to find a key, which is hidden inside the body of one of the students, so they can "graduate." Once a student has graduated, nobody in the school remembers them.
I had heard this was a good title, and bought the first three volumes when I was in Japan. They sat on my shelf for months, and then I picked them up and read them practically all in one sitting...and then immediately ordered the next three from Amazon decided it might be a good idea to change my monthly manga order to the end of the month. I am totally obsessed with the story and can't wait for the next volume. Usually, I'm the type of person who sits back and waits to see what will happen next, but with this I can't help but try to figure out what's going on and spend a bunch of time thinking up crazy theories. Also, there's a love triangle, which might make me inclined to knock a few points of the score, but it's really there to illustrate Mashiro's gender confusion, so I can understand. (And I think Mashiro's confusion is understandable, given his situation, so I sympathize with him, even when he kind of acts like a jerk.) He wants to keep living as a man, but can't deny that he's drawn to Sou (and acts like a girl around him, heh).
...I seriously could go all day talking about this series, the parts I hate (when Mashiro is an idiot and Kureha needs to get lost and so does Sou's sister and what the fuck is going on anyway) and the parts I love (the mysteries and symbolism and interesting views on gender issues and Sou making me cry and MASHIRO + SOU 4EVA OMG), so I think it's best to stop here.