the outside doors where locked, the thing simply blew them out with their gun and climbed through.
It could have simply drove through them also, there is no way to stop someone like this
Stop them, no, but potentially buy time with thicker glass or steel doors with windows too small to climb through.
I noticed the shooter stepped through the broken door instead of opening it. Since she was a former student, I wonder if she suspected that opening the door would trip the fire alarm. It will be interesting to see if the door alarms included sensors for broken glass.
I also wonder if the school was wired inside and out for gunfire detection such as Shot Spotter.