[Joseph Smith]

Here's my advice: first call a meeting with all current mentors and the school administration (minus coach Z.) Explain the situation, and how you feel the new direction is negatively affecting the team. Decide how you would like to re-organize, whether by choosing a new coach or having the mentors work together to make team decisions. Once you're all on the same page, have a meeting with everyone including coach Z and let the school administration explain the team's concerns and the new organization. The fact of the matter is, one mentor (who hasn't been around very long) should not be able to "decide" that he is in charge of the whole thing. If the majority of the team disagrees with him, then the team can choose to make a change.