off the top of my head, except for Adam Lanza in the Newtown shooting and Laurie Dann in Illinois back 20 years ago, most school shootings involved shooters who were current students at the school. I'm not sure there's any reason to think that an unstable 12 or 15 year old of the future is going to choose a different target, unless you are thinking of these as copy-cat crimes. They choose their school because it's the place they know best.
The motivation of a young person going in to their school with a gun is different than the motivation of someone like the Aurora movie theater shooter or Jared Loughner in Arizona.
True, and for troubled youth their school is often the place they experience much of their trauma. I suppose if a youth is going to go on a shooting rampage their school will always be a popular target.
Schools are fairly open places but I see that changing already and assume they will become more and more secure in the coming years.