How do I go about talking to the principal?

Wait-- did you all take the AP Physics exam? And do well on it?

Then why on earth are you retaking the course?

And if you didn’t take the exam or do well on it, then why on earth are you hoping to take the second course?

To answer your question: two or three of you need to make an appointment with the principal’s secretary. Go in calmly, like an adult, and make your case rationally.