John,
I agree that most schools have policies in place to keep strangers out. One of the problems is that schools have a lot of doors. Some i am familiar with have between 6 to 10, just counting the main doors and doors at ends of halls and wings. You can lock these doors and give access control badges or keys to staff, but unless you have an officer at each and every door it is just not feasible to control access all the time. Kids out at recess need to go to the bathroom, parents need to drop off forgotten lunches, cafeterias need food deliveries, custodians need cleaning supplies, etc. If you walked up to a locked door at an elementary school and waited a few minutes you can be pretty sure that a student would walk by soon. How many kids would ignore a friendly looking adult knocking at the door asking to be let in?
Even schools that buzz visitors in (like Sandy Hook) normally leave the doors open right before school starts and at the end of the day. What school can afford to check the ID of every student, parent and visitor when hundreds of kids and dozens of teachers, parents and visitors are arriving in a very short time?
What is happening is that most schools (in my limited experience) seem to be making a reasonable effort to control access, but given their financial and public use constraints, they are never going to be complete. Since they can't say, "we are keeping your kids as safe as our budget allows, and by the way, your kid is more likely to be shot off school grounds" they are an easy target for reporters.
There are ~ 55 million K-12 students. Take that times 180 and you get 9,900,000,000 student/school days. Divide that by 600,000 violent incidents and you get 1 incident per 16,500 days.
I honestly don't know what schools should do. They can't tell parents to just chill out, the odds are in your kids favor, but they can't meet the often impossible demands placed on them by the public all riled up by stories of horrific school shootings. I don't even have children, but to this day I am sickened by the thought of somebody shooting first graders so I can't even imagine how parents feel about this. Unfortunately until schools have unlimited budgets and omniscient staff and administration - and every single person who is unbalanced gets recognized and treated - school shootings will continue. Jeesh, talk about a sucky way to end a comment.