Lock doors

I have been waiting for someone to answer the question as to how the Texas killer got into the building and then into the classroom. I go to 14 different high schools and they all lock the doors when school starts and they have a person hired to watch one door and everyone must enter through that door. Once in class the room doors are kept locked. The latest report say the person carrying a rifle entered the school and then entered a classroom. Why weren't the door locked? As reports come out this question may be answered but as yet it has not.