Nothing too unexpected there. Ishi died before the whole business with Justice Robo. Celes and Yamada planned the whole Robo thing. Celes leads the group to Ishimaru's body; meanwhile, Yamada fakes death. Celes finds a way to get everyone out of the room, Yamada rises up and goes to the third floor. Takes Ishimaru's body along when everyone's down in the infirmary. Goes into the art room.
Now either Yama is still alive (doubt it given that we allegedly see his last moments, but who knows. It's hard to not die instantaneously by a strike to the head, so the whole 'final words' thing sounds fake) or either Celes/Hagakure killed him while everyone was searching the floor for the bodies. I'm going to say Celes for now, but again, it's a bit simple, so I'm not too confident in my theory right now.
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When you have nothing to say, quote yourself ~ Solfadore