This is everything the main anime should've been but wasn't. I actually feel like this author could've made a way, way better manga if he had all the characters from the current one, removed their edgy pasts and inserted them into a weird high school setting like this OVA had. Seriously, the main story is pure, unfiltered garbage. Well, this OVA isn't that good either, but at least it's much better than the real deal. For starters, it properly knows it's a gag comedy and nothing else. If the author had kept true to the premise of his own story without edgy, half-arsed dark pasts
and subpar drama mixed with boring comedic relief and dragged-out battles, it would've been so great, prison setting or not.
So, what is this OVA about? Well, nothing much, as it should be. It's all just for laughs. Nothing complicated going on as it parodies the main anime by putting similar events and the same characters into a high school setting where prisoners become students and guards/prison staff becomes teachers and school staff(although there are exceptions to this rule, like hajime being a student instead of a teacher, not that he acts differently from his guard self). I wouldn't say it's hilarious, but at least it managed to make me smile and rinse away the bad taste of the butchered and minced main anime. A good ending to a crappy experience. Recommended watch, although I will never watch or read anything by this author again. Once is enough to know the style of the author and I personally find his style to be almost offensive(things like not staying true to the setting it was supposed to have is something I despise, especially if the author isn't talented enough to connect the parts properly).