Will tutoring calculus be worthwhile experience?
I am not an academician and I have little idea as to what to look for when hiring a lecturer, though I had the opportunity to sit through a few of those demonstration lectures from prospective candidates and also go through their feedback forms given to other lecturers.
From this little insight, I believe that any experience is better than no experience. After all, you would be teaching the same students a few years later in college if you become a lecturer.
More importantly, leaving aside what your PhD committee or the hiring committee feels about you it would be a good experience and I feel that you will learn a great deal in teaching. Teaching is an art and you are honing it at a level when it matters the most.
As for leveraging it in your career as an academician, in your future interviews do not just mention that you had experience teaching high school students. Instead, focus on the challenging scenarios which arose while teaching and how you managed it. And then explain how that experience would be useful for you as a lecturer.
A good lecturer has his or own style which is inherited from their senior colleagues and gets mutated through experiences. You are doing good by being proactive and training yourself beforehand.
Finally, if there is a competition for the position of TA in your school, I am sure this experience will benefit you.