The school has electric locks and the doors are supposed to be locked at all times. This day however the doors were not locked.
Things that make you say mmmmm.
Because of fire regulations doors that are locked from the outside have to have mechanisms to open them from the inside. You see these all the time look for the push bars on the inside of the doors. The doors are also not solid they have Windows breaking one of those and reaching inside to push the emergency bar is a way to open a school door. Some not all have glass with chicken wire in them to prevent access like a jail cell visitation room glass. He also could have just shot the lock out he had hundreds of rounds and a rifle more than capable of shooting the locking mechanism to pieces. Then again I could be a janitor didn’t do their job and lock that door.
My understanding is that the school was given some warning in advance and or the murderer crashing the truck outside of the school caused the armed resource officer to come out of the back door to “investigate”. At that point he and Ramos engaged although it is not clear the officer fired at Ramos, but that Ramos fired at him and was able to force himself through the door that the armed resource officer had exited.