This is just speculation based on the clues provided, but I think that the school campus has been completely quarantined from the rest of the world by their government. Since the last message the school gave out was to not go outside, giving out false hope that they will be "rescued" but won't.
I think it can be deduced that the government thinks this place is a lost cause and are waiting till every last person is dead to burn the place down or something, or just completely shut it off from the rest of the world permanently.
By the time the MC's get to the entrance, I'm betting they'll be blocked off by a huge fucking wall or by soldiers loaded with militarized guns, ~life restart~ this is so fucking sad to even think about it.
（￣へ￣）
I just binged this whole thing up till the recent chapter, time for bed
Genuinely felt confused when chapter 65 jumped to 70 ...