Each classroom has opposite doors (inside doors and outside doors), and a panic button on the wall.
And, at least in elementary, each kid has a panic fob (fob with a string like thingy, that when separated, sounds a very loud and ear piercing sound), that is detached and activated very easily...
(and very annoying)
(After lots of searching because I don't know what the pole is called, I found a tiny picture of one):
On the other hand, there isn't a gun issue in Japan, and the pole would have been no use back here in the states against a gun.
BUT, a gun in the teacher's desk would certainly help...