My daughter had this same question and she emailed her AFA admissions counselor.....here was her response:
"You should ask your guidance counselor, principal or a science teacher (chemistry or physics). It has to be someone who is employed at your high school. You may ask your CAP instructor to write one of the optional letters of recommendation though."
So, that makes sense.....they are looking for someone AT THE HIGH SCHOOL (or at the college in the case of ROTC)....so AT THE SCHOOL. And, CAP is not associated with the school.