Which tests for an over achieving middle schooler.

[Carnation]

University of California does NOT accept CLEP exams for credit.
Some homeschoolers still take CLEPs just to support their "Mommy grades" on their homeschool transcript.

The California State University system DOES take CLEP exams and you could essentially CLEP out of almost all of your general ed requirements if you are aiming for a cheaper college degree.
If your student takes the CLEP exams before graduating from high school, they can still qualify for freshmen scholarships but gain advanced standing from the CLEP credits.

In general, you want to start CLEPs in subjects your student excels at or is passionate about.
For instance I have a son who loves history and government so that's where he is starting.
So whatever subject is your student's passion, let them study it in depth and then take practice tests to see if they have the knowledge to gain college credit.

Edited to add: If your student is younger than 13 years old, then it can take a week to get them signed up to take an exam.
https://clep.collegeboard.org/parents-info

Best wishes.