Ours has that policy but it would be *sooo* easy to work around it, that I don't consider it real secure.
I've taken the kids birthday treats in or have to go to a meeting and just walk in the door that's closer to the parking lot rather than walking all the way around to the front since it was always before school & anyone can come & go in & out those doors. They have patrols there that will open the door for you! We basically have 2 main doors to the school, once the tardy bell rings than only the main doors in the front by the office are open to the lobby area & you have to be buzzed into the school area.
You are supposed to sign in & get a visitor badge but a lot of times for the drop off treat thing, parents don't & I will be taking my 3 year old for speech there soon, I'm coming in via a locked door, the speech teacher will open it for me & I won't be going to sign in since I'm not even in the area of the office. I'm not sure I am expected to stay the entire time or come back in an hour.
The kids can *always* get out though. I would have a big issue if kids weren't able to get out in an emergency.