One door in one door out. Metal detectors and a guard. It makes a statement that you are not going to easily be able to wreak havoc. Most of these crazy kids shoot up the school because it’s easy.
Sounds great if you wish to ignore Fire codes, and essential run the place like a prison.
And the costs, Each guard will be unionized and sit on their asses much of the day.(See Parkland, for an example)
In my opinion it's better to have smaller schools(yes more money) with a few 'Resource Officers who the shooter would have to account for and NOT know where he/she would be.
To easy to go though an unlocked/unsecured door and start shooting
I'm open for a few properly trained AND willing teachers to conceal carry in the classrooms.