Just to reiterate, kids who audit college courses can still take exams and get credit via CLEP , DANTES, or other subject test offered by the college, as well as AP exams offered through the College Board. SAT subject tests might also be used to prove mastery, and perhaps help with placement.

A few things make it easier for a young student to be admitted for credit:
1. high ACT or SAT scores
2. completion of all/most high school material available in a subject
3. confidence and self advocacy