I don't think the show is trying to be seen as having a central source of tension or threat. The whole point of this show are the personal stories of the women in this minimum security prison. Anybody coming in expecting OZ with tits is going to be disappointed.
In a way, you hit the nail on the head with comparing it to high school. That's exactly what it's like in a way, and I think that's an intentional move on the part of Jenji, It's true to the dull reality of most minimum security prisons.
It's not insomuch that I'm expecting Oz (never seen it btw) or anything on par with, say, The Wire. Those kinds of shows are pretty exceptionally rare and it would be unfair to expect Netflix or anyone else to immediately start being able to immediately knocking out a show of that caliber.
I was, however, expecting the prison component to provide something more than a bare minimum backdrop to events. As is, the prison setting doesn't do much. There aren't any particular insights that the setting brings; it's effectively a loose backdrop just to put all the characters together. I mean, Mendes has the equivalent threatening aura of Coach Cutlip from the Wonder Years. To me, it just sort of undermines the basic setting and premise of the show.
But even that would be passable, and I would be inclined to agree with your point above except that the personal stories get short shrift. Early on, the show sets it up as a key ingredient of the show's format, but then it sort of just drops it and occasionally brings it back. Even when you get those flashbacks, they're bare minimum exposition pieces (e.g., this girl wanted to fit in, so she engaged in some petty theft and now she's here). Some key characters never even get the treatment, and there's just an inconsistency and lack of depth to those bits. I think the show might actually have benefited from scaling back the number of regular characters, so that the writers could've tried to give a few of the tentpole characters more screen time and a little more depth.
All that said, it's not a terrible show, it's just not a particularly good one. Admittedly, the bulk of the reason I'm even watching is because I have a genuine curiosity to see how Netflix's gamble on original content would pan out. If Orange is the New Black was a show by other means (i.e., traditional TV), I probably wouldn't even have tried to finish it unless I was just bored after work during the week or something.