The APS has gobs of interesting data on physics education & careers, if you haven’t found them yet.
https://www.aps.org/careers/statistics/index.cfm
I have a D who was a physics major (condensed matter, though) and is a PhD student now. PM me if you want to chat more.
@mikemac We live in MA
@intparent Thank you- I just may when I have some intelligent questions. A little out of my league on this but trying to catch up!
I was, too. 
Oh, and some schools offer PGRE prep. My kid’s college did, although she also self studied. (And some prepare you academically more fully for the PGRE).
Ok. I had a rec if you are in CA but it does not apply since it is a public U
Physics professor here…
The Physics GRE test covers undergraduate-level topics, so graduate courses are not necessary to do well on the test. Just like the MCAT, one does have to study a wide range of topics and practice working efficiently. Different physics departments offer different amounts of help. Some do nothing, others encourage study groups, and a few organize informal test prep sessions. Students who prep for the test usually do much better than those who do not.
As others have noted, there are few materials available and no Kaplan prep courses. The University of Colorado posts some old Physics GRE tests on its website for practice.
https://www.colorado.edu/physics/GREExams/
Besides those practice tests I recommend that students make sure that they understand EVERYTHING in a good first-year physics textbook that uses calculus.
The Physics GRE is not the most important part of a physics grad school application. Most graduate departments use it only to cull out the least qualified applicants. Far more important are research experience and letters of recommendation that attest to the applicant’s research potential.
A high school student selecting colleges should not worry about the Physics GRE or applying to graduate school now. There are lots of possibilities for a STEM oriented student, so she may not be interested in physics graduate school in four years. To keep her options open I would prioritize finding a college where she can thrive and which provides good access to research as an undergraduate. That might be a liberal arts college or it might be an R1 university.
@Fifty Thank you for your insight! Yes, right now we will focus on finding a good fit with research opportunities and worry about grad school later!