On the topic from a year or so ago...
In the anime: Shiroe, Tohya & Rudy are the only ones who explicitly say anything about knowing, though apparently Tohya thought it was obvious. Naotsugu & Marie definitely don't know (Marie called Tet a "cute girl" or something like that and seemed worried about her, despite saying she wasn't). Anyone else, it could go either way.
In the light novel: Only Shiroe mentions it. The kids don't really have any interactions with Tet. At one point during the kid's saga, Tohya is thinking about the girls in the guild and comparing his relationship with them to that with Dariella (or at least that's my understanding of it...). He mentions Tet but says she doesn't count (or something like that) and moves on to someone else. Why she doesn't count, isn't said. He included her in his list of girls at least. Maybe he just doesn't really know her (after all, she mainly seems to keep herself glued to Nao. Even in the anime she didn't really interact with the kids much).
On Tygari's comment...
Tetora is a girl, at least physically (despite some people's arguments), mentally, it's unknown what goes through her head. I'm pretty sure Roderick said that personalities were changing to match their avatars, so Tet should be becoming more of a girl, in terms of personality, the longer she's in that body. I think that the scene of her fixing her nails in the anime was to show this. She's been a girl for almost a year by the end of Season 2 (vol. 10). She uses the male version of "I", but that might be more from habit and not really an indication that she identifies as a boy. It's quite possible she's not entirely sure what to say if someone did ask her what she was. XD She does call herself a bishoujo though...