DISREGARD..... figured out that its hakomari after searching my post history.
I remember it starting out with a classroom setting, something happens and the guy dies (maybe a female student kills him). Anyways he ends up back in class and no one else but him remembers anything and it keeps repeating in cycles until he comes to the realization that the girl is cognizant of what's happening too. Eventually they break out of that for the first volume and something else happens in the next few. I seem to remember something about a cube as well but I'm not sure. Any help would be appreciated.