I think she only blamed him because he had the power to save more people instead of taking "The easy way out", since the only person with the clearance to not only know about the bunker but get to the bunker (probably before, but possibly during the zombie apocolypse) and lock themselves behind a door with a pass code lock would have to be the principal - or someone with that level of power - and they would've been briefed beforehand. Either that, or it was a teacher that read the survival guide in advance and holed himself up instead of alerting/helping anybody else.