The yeshiva that my kids go to keeps the doors locked. There is a security guard who needs to buzz you in, and he sits right by the door, so he can escort you somewhere if necessary.
If he’s not there, like if he’s using the restroom, you’ll wait. I’ve had to wait before to be buzzed in because he was away from the desk for a moment.
The school also has regular drills with the local police to know how to handle a shooter in the school.