So what’s going on here?
When the initial outbreaks occurred, a very different - yet very similar - calamity occurred in FPS. I’ll leave explaining the details to FPS, but suffice it to say that there’s a different strain of zombie loose on the islands.
When this occurred, Shiro Academy was locked down. Oddly enough, the Academy had been designed with defense in mind, and in the earliest days of the outbreak it was more broadly fortified (albeit with limited success as attacks and the loss of supplies resulted in a very mixed perimeter). Despite this, it has held out ever since.
Life inside the Academy during the apocalypse is very different from its normal state. As a place as safe as any, the site has become a haven of thousands of refugees, both in organized groups (such as the evacuation of embassies) and from ragtag survivors wandering in. Personal space is tight, supplies are limited, and tension is high on the grounds. The Academy has effectively ceased to operate as an educational institute as refugees and former students alike try to make ends meet and stay organized and sane within.
Ironically, since it is as safe a site as any (which admittedly isn’t saying much) many nations have made no attempts to evacuate their students and other nationals. Unlike previous Shiro threads, it should be easy enough to justify the presence of just about any character you want to be present. A lot of different sorts of people have wound up holed up in Shiro for a lot of reasons, ranging from former enrollment to evacuation from somewhere else to just being in the wrong place at the wrong time and having nowhere else to go.