I found out yesterday that our local community college offers AP, ACT, and SAT testing. Who knew! Perhaps you could look into that as an option?
Our high school is pretty liberal about letting anyone take AP tests IF they only require the proctor and the exam booklet. If the test requires “artifacts” (AP CSP), or playing/composing/listening to music (AP Music Theory), or any kind of project or research submission, then no deal (unless you are enrolled in the high school’s AP class).