The only way that you will not go to primary is if you totally fail out or can't pass the pool training. Each class has one final exam, and you have to pass it with an 80%. If you fail a test, you get a pink sheet and you take it again the next day. The second time you fail a test, you usually get rolled into the next class. The third failure will probably send you to an academic review board to find out what the problem is.
API is a pass/fail deal. If you get an 80% on every test nobody will say anything to you and you will go right on to your primary squadron just like you would have if you had made 100's on every test. Your final API grade goes in your training jacket, but it does not count toward your NSS or selection of platform after primary.
People do actually fail out of API. I don't know how, but it seems like there is one in every class who just can't cut it. The one from my class was lazy and didn't really want to fly anyway, so she just took her 3 pink sheets and went on to something else. Just make sure you don't fail anything so you don't have to show up to your primary squadron with a pink sheet in your record. Other than that, don't worry to much. API is easy.