Apparently the anonymous posters are asking for specific techniques to deal with their problem, so here are two:
(1) All of the students should agree that they will all attend the sensei's classes, and that none will attend the senior student's classes.
(2) Someone should contact the former sempai and ask him/her for advice/assistance.
Number 2 may help in terms of advice - but it's still the people involved who will have to step up and act.
Number 1 sounds like a form of shunning, and it's exactly the kind of passive aggressive response that's popular in many places, but which I strongly dislike. Whatever happened to just talking directly to the people involved?
I really think that it's not rocket science, and that there's no need to cook up a scheme to get the point across - just talk to them and deal with it. In the end I think that it works out better.