How is the Japanese school security like?
Rather than school security I think it is more than neighborhood security
Just like Sue Umezaki said in most places the community, especially the retired citizens, would be the ones patrolling for outsiders, as well as look out for fires and robberies. Also there are a lot of houses that have a sticker to show that it can act as a safe house if and when children (or women) feel they are being followed.
As the schools per se, some schools may have a guard, most don't. If there is someone unknown inside the school perimeters the children are bound to inform a teacher who in turn will call the police.
A good thing in Japan, the police will not take more than 20 minutes to arrive.(mostly they will arrive within 15 minutes of receiving the call)