Out schools have had locked doors as long as my kids have been in school (9+ years). At the high school, you can walk into the front doors but the second set of doors are locked, so you have to turn and go into the office to get anywhere. At the other schools, the exterior doors are locked and you either knock or hit the buzzer and someone inside remotely unlocks the door, letting you in. You then have to go to the office and state your business. If you're volunteering, you have to sign in and wear a visitor's badge.
Since CT, the only change I've noticed is that when I dismissed my youngest early today, they didn't just buzz me in after I knocked. I had to state who I was and why I was there before they would buzz me in, which is fine by me. We did get a notice from the superintendant stating that on Sunday, the school principals, assistant principals, superintendant, police and fire departments met to review safety plans. They stated that they have had lockdown plans in place already and that although they don't do them with the students, the teachers and staff are already trained. They added that they will make further changes to tighten things up more but that they would not release the details of their plans to the public, which makes sense to me. No need to advertise to a would-be assailant what your response times and emergency plans are or where all of the hiding places are. That said, I do hope that they eventually do some kind of drills with the kids, along the lines of fire drills, so they understand the importance of following directions in an emergency very carefully.
ETA: The middle school and high school students and staff have always had to wear their IDs on a lanyard at all times. No ID means you don't belong in the building. In the younger schools, they are now requiring all staff to wear their ID at all times as well.
And I have to add that they would NOT have let in a guy wearing body armor and carring three guns, including a rifle. Let's be reasonable. The security measures in place absolutely saved lives. That he had to blast his way through the front door instead of just waltzing in silently gave early notice for someone to turn on the PA system and call the police. The noise of those shots and the PA gave the teachers and staff a few extra seconds, minutes, who knows, to lock their doors and hide the students. Hidden students in one classroom escaped slaughter, and he killed himself when the police arrived. Given the speed with which he worked and the amount of ammunition he had, every second counted. It didn't prevent 26 students and staff from dying, but it did save perhaps dozens or hundreds more.