We don't assign titles that distinguish between levels of mentorship. We do have specific "Lead" positions (such as "Lead Teacher" and "Lead Electrical Mentor") and heads of various committees, and we have a defined Steering Committee (which is responsible for things that on some teams is done by a single Team Leader), but basically a Mentor is a Mentor.
We recently had some discussion among the adult leadership about whether team members who had just graduated from High School should be accepted as mentors (the consensus was that we want them to take a year "off" to give a definite break between student and mentor roles). The term "Junior Mentor" was used casually as a shorthand reference, but nobody intended for it to become a real title.
The closest thing to "Senior Mentor" we have is "Team Grandma".
