Sakamoto lies to the class that he's going to participate in a project which includes him going to mars. This of course is a lie, and he is going to die in the near future. Some spectate that he has an illness/cancer which makes this year his final year of his life. (He is allowed to speak at the graduation ceremony even though he is in his first year, and at the end, his final regret was not knowing the exact number of holes in the pharmacy, indicating that he was at the pharmacy regularly.)
This of course is just my speculation. Some people also point out that on the cover page, he is like a corpse which foreshadows his death.
Adding onto my previous reply, throughout the entire manga, Sakamoto does not show any emotion towards his personal life and does not hold a relationship with other classmates other than that one guy. He probably did not want to make them sad when the news of him leaving is here, therefore he distanced himself from everyone else. There is little to know about Sakamoto's personal life which some say indicates that there was nothing really happening in his life or is constant and is not worth showing. The last scene shows Sakamoto looking back to the classroom window where everyone is telling him how they won't forget him, showing a sad expression. This is the first time throughout the entire series where Sakamoto shows any kind of visible emotion. The other part is where his face is covered in pie, some say it's because he's crying. This is just my take on the ending. I can elaborate more on it if it helps.
Other speculations are :
Sakamoto is an alien and is returning to his hometown in mars.
What's with the ending? I don't get it ._. CAN SOMEONE EXPLAIN PLEASE!