Our school is pretty lax (British International school in Asia)
Parents and staff have passes, and visitors need to leave id with the security gate to get a visitors pass but after that they are pretty much free to go where they like.
I don't like it.
One of our staff members had a row with a particularly pushy parent who was just hanging around the Y3/4 corridor. She politely asked if she could help, mum said no I'm just looking around, staff said, well, you need to go back downstairs, there is no real need for you to be here, staff got a mouthful and a complaint made against her.
I know becuase the mum was complaining to me as a governor. I was completely in the staff's camp, but had to smile and nod.
Our school is difficult to police though, once through one of the four gates, it's very open plan. Parents do need access to say, school office, uniform shop, bus office etc, but not classrooms unless they are volunteering for reading etc.