Obviously Nico wouldn't even have Maki in the shot. Well, not her face at least.
I think all of them but NicoMaki and KotoUmiHonk have the phones the proper way. The reason Nico's holding hers the wrong way is probably just so that we, the viewers, can see what the picture is of (namely, herself and Maki's boob). I dunno what's up with the 2nd years though. I can't even tell what's going on with that screen.