You do not have to have an AP class to take the AP exam. Most schools will allow students who are not enrolled in a class to take the exam. Some schools discourage it, so be sure to ask your school.
Since your school does not offer AP Biology, you may need to take the AP exam at another high school. Sometimes a school might offer just one exam, but usually not. We helped ~100 schools register students for the exams online and many of those schools administer exams to kids who are not there students. Most schools register their students Jan-March (not all of them do it that entire time) so start asking around after Christmas. A counselor at your school might be able to point you in the right direction.
Get your hands on one of the practice exams to see how well you do. This will give you a good idea on whether or not taking the exam is a good idea.
The main difference between the AP and the SAT II is that a good score on the AP exam may earn you "transfer" credit in college that will count towards graduation. This can save you a lot on tuition. However, you may already be achieving this by taking the class at community college.
Mike - Total Registration, LLC
http://www.TotalRegistration.net - Helping high schools simplify the AP exam registration process by registering students for the exams online.