I'm gonna put this theory out there, right now:
From the second you turned on the game, you were already in Eternum.
The school. The quirky friend. The bullies. All the game. I feel like it was a meta gag/hint, that the game is called what the game in the game is called. You're playing Eternum. The whole thing is that.
So here's the thing. Maybe you playing it for the first time is not real. Because if you played Once in a Lifetime, they already said this game was being made. Again, Meta. So fastforward to now and you got a SAO-ish game with a needle being put in your arm with sex in the game? Naw man, it was all the game. Start to finish. The only thing that will not be the game, is like the final chapter of the game with the reveal.