To reiterate what Arby said, in a more blunt sort of way, you did one of 2 things. You either; 1: had a daydream, or 2: fell asleep altogether. The fact that you saw the classroom you were in was because it was of your mind, obvoiusly, since you were in the class at the time. I call it a stronger kind of "Day Residue". Anyway, this really isn't a technique, to be honest....