Ah, the wonders of press coverage
... According to The Japan Times
, the knife was only 15 cm long, and only one of the dead kids was a girl, not the other way round. Not that it matters in the face of such a tragedy.
Of more relevance are details about the killer and his motives; the one cited above shouldn't be take at face value. After his arrest, he made a lot of totally contradicting statements, including one in which he denied ever entering the school. Apparently he was being treated in a nearby mental hospital, but hadn't been deemd truly dangerous. He also seems to have overdosed some of his medication prior to the killings.
One of the most chilling realizations about this even is that there really isn't any way it could have been prevented. Oh yeah, everyone's now saying how the hospital should have realized that the man was a potental killer, but that's just hindsight. Even more ridiculous are the cries for improved security - do they really want every school in the country fortified using money that could go into better education instead, just to defend against the off chance of a random nutcase?
The truth is: there is no such thing as total safety, not in a society you'd feel comfortable living in.