I lived in Temple for a while (down the road from Killeen, Belton etc). I also have friends who work in all of the schools in that area including Harker Heights, Shoemaker, Killeen HS, Belton HS, and others.
My impression from my own experience and talking with my friends, is that Killeen schools can be unstable due to the transient nature of their clientele. This is not to say that they are bad folks. It's just that many of them have circumstances in their lives, which we do not have to deal with. Many military parents are re-stationed after only a few years, so it is difficult for kids to get adjusted to the area before they have to move again. This creates a sort of constant uncertainty in the air and I can possibly see this angst transferring to aggression in the kids. Shoemaker is the hardest hit, since it is basically next door to Ft. Hood.
Harker Heights has the same problems, but to a lesser extent. Many of the military families who attend Harker Heights are more permanent fixtures on base and more likely to stick around, but it is still in the Killeen School District and faces some of the same challenges that the other 3 HS's do.
Belton is a bit more rural and much less affected by Ft. Hood. This school district is highly regarded in the Central Texas area and would be a very good place to start taking a closer look.
If you would like more information about this part of Texas, please feel free to contact me through my profile. I am pretty comfortable discussing in detail.
Otherwise, Good Luck.