Just teach whoever needs help.
It depends on the dojo. In our dojo, the shihan is a japanese. Even if you are a high ranking aikidoka, only the shihan can teach whoever needs help. If the class suddenly gets too loud, we'll get a telling off and shihan would say: "You don't teach, I'm teacher!". I guess its a japanese thing? I heard you're not even allowed to speak during practice in some dojos.