When you begin Spelling Power, you start by assessing what how well your child spells. Then you start your child at that level. When my child was in 3rd grade, he tested at level G (about 7th grade level,) so I started him there. So the words are exactly as hard as they should be for your particular child regardless of how well, or how poorly, your child spells. There is an alphabetical list of 5000 words in Spelling Power.If you come across a misspelled word in your child's work, you can look it up on the list. It tells you what grade level that word is, and if it is in a SP level list or not. That helps you decide if you want to add it to their spelling list or not. If the word is a 9th grade word, and your child is in 2nd grade, you probably don't want to add it to their spelling list. But if it is a 6th grade word, and your child is in 9th grade, then yes, you would include the word in their spelling list.
Anyone can take an AP test, even if they have not taken an AP course. The tests are really hard, really long, and the student needs to be prepared.