Sempai is descriptive of one half of a relationship between two people, not of an absolute position.
Considering that the term applies to all members of the dojo except one, "sempai" isn't very helpful. And it's another one of those Japanese words that serve as jargon at best and a private language at worst. But I suppose that should be that the term applies to all members of the martial arts studio except one.
I seem to be going in the wrong direction.