So when I saw my students waiting outside again I popped my head in the door and said to the Prof:
"Hi, excuse me, I have class in here."
To which he replied, "yes, of course, take a seat."
It was all I could do to hold back my giggle, "no, no, I'm the one teaching class here today, my class starts right now."
He turned bright red and said that he would finish up soon so I could have the room.
I mean I know I don't dress to the nines to teach, nor am I in my 40's or 50's but come on, I had a huge handful of whiteboard markers and my keys... usually that's a dead give away!