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All I can say is WOW!
As far as atmosphere, mystery and horror go, Umezu lived up to his reputation of being the master of horror manga. Umezu showed as well a deep understanding of a child's mentality: they can be unreasonable, they can be selfish, they can be bratty, their innocence could belie a cruelty and they see the world in black and white. Yu was also so adorable, and a great addition to this rather dark title. And Sho's mother was just awesome; I was quite touched by Umezu's portrayal of the power of a mother's love and how she would go to any length to bring her son back.
I also thought the ending was bittersweet and beautiful; it may was not what we've been expecting and hoping for, but it left us with something to think about.
As awesome as this manga is, there were a few things towards the end that I felt it could've been handled better. Like how the mutants, who were portrayed to hate humans and kill them with no second thought, immediately retreated from the school after a corny speech from Sho, and it was never elaborated on how the bully girl turned into an old woman. Also, I was quite dissatisfied with how rushed the part with Otomo's confession was: when everyone thought it was Sho, who risked his life for them and struggled to keep them together, everyone wanted to kill him, but when it was Otomo who only caused trouble everyone suddenly became so understanding.
But even with those flaws, The Drifting Classroom is highly recommended, and even after four decades it's still better than the majority of the newer titles out there.