That's a great video.
Funniest line: "Wow, I thought you were a teacher." That certainly helped him. He did not 'look' like a threat so people were less likely to confront him.
The reality is - closing every entrance - confronting every person - makes schools feel like prisons (maybe that's what they have become - see the absurd school crime stats) but it's a heavy burden on everyone to enforce that.
We do a ton of work in K12 school districts in NY and it is a joke at times how easy it is to get into buildings in some really scary neighborhoods. We typically find that the level of crime within 500' of the school is inversely proportional to how hard it is to get in the building. Nice areas, impossible to get in the school. Bad areas, every door is propped open. Its really sad, but this is America.
The weakest link in any system is the human element no matter what the technology. We always told customers that in addition to security cameras/access control/etc you also have to train your people in proper security policies.