School doors in high school actually tend to work pretty well for hiding. The doors have two locks, one that isn't on the knob. The doors are very thick, the windows are very thin and small, so there is probably a part of the classroom that the shooter wouldn't be able to reach, or it would just not be worth the hassle to even try to get in because of how short of a response time there is. I believe in Parkland most of the deaths game from one classroom because he was able to do that one first, so no one was hiding yet. Everyone else was generally okay