[Chak]

I'm just a young student with little experience in the world, but I know what I'd do if this happens to me.
I would quietly make a petition basically stating that you don't like where this team is going. From the original post, it seems like such a petition would gather lots of support. Then gather a few core veterans from the team and confront Z in a group, with the petition representing those not there. Then it's not just a single member "revolting", it's the entire team collectively rejecting Z's ideas. After that you guys can have a conversation with Z, and figure out something.
I think you absolutely should not remain quiet. FIRST is for students, not for mentors. If the students disagree about how the team is run, their voices have to be heard. Heck, the opinions of the students should always be part of the decision process, especially for such a big shift like this one.