For us it's normally ok. Fence around most of the school. One side is a brick wall shared with the highway, but the other ones are all easily climbable. There isn't really any security measure in place, except a crossing guard at the front entrance where all the cars come in and out (which makes for an INSANE traffic jam every morning, being near a highway and all.)
only thing is our school's "Code Central" and "Code Blue" procedures.
"Code Blue" is for emergencies. we went into it a few years ago when some armed person robbed a bank down the street. Basically, you're all confined to the buildings, and all the outside doors are locked. That's all fine and dandy, cause most of the classrooms are inside one building or another, all with running water, bathrooms, etc... unless you're in the portables, in which case you're screwed if you hafta pee, but at least you've got air conditioning.
"Code Central" is something really dumb. In the event of a biological or nuclear attack, we all congregate (all 6 grades or so) in the gym/main office complex, which is then sealed airtight. Somehow, they calculated that there would be enough oxygen in there for all 1000 of us to breathe for a day or so, after which we have to either sit there and suffocate, or open the doors and breathe anthrax, radioactive iodine, or whatever else they were trying to protect us from.
Go fig.