Tell the Head Sensei "Excuse me, I believe you're in my seat!" THAT is a surefire way to be told where to go
In truth I would introduce myself to the instructor at the start and give a brief background of me (rank, style, teacher). I would then proceed to sit at the end of the line. The sensei can then choose to move me up or let me stay there. You know what you are.
I have seen the grandson of the creator our art sit at the end of a black belt line when he knew VERY well that he was ranked higher than other people there (modest and wouldn't move up when told (more or less) "hey, you out rank me you should go before me." He has replied that's ok, I know what rank I am or something very similar. He was in his early 20's at the time and I rember that being a very mature and humble remark.