To do really well on the SAT II's, all you need to have is discipline. If you can force yourself to study for long periods of time, and find a lot of practice material, then you'll end up in the same place as the people who take a class. All a class does is ensure you spend the time studying; there's no secrets being passed out or any answers being given away. If you can't make a schedule on your own that you'll stick to, or you just like to day dream after every few minutes, then a class is the way to go. You really can do it all on your own though, as long as you can stay focused.

If you take the courses in high school (i.e. AP/regular physics, biology, and regular chemistry), then you already know everything you need for the exams. It's just a matter of being asked the same material in more ways than one. Barron's is really good for physics, and Kaplan worked for me for biology and chemistry.

Do as many questions as you possibly can, and you can't go wrong. It's not about memorization, just practice. If you finish the practice questions in your book, then get another book and finish that one too.

Shifa's requirement used to be 700 in Physics, Biology (M), and Chemistry. They only ask for 650 now, but they get plenty of applicants who are well over the requirement.