You have to test out, I'm doing the same thing for Alg II so my sophomore year I will be in Pre-Calc. Basically all you have to do is schedule a meeting with you assigned counselor. He/She will give you a little form to fill out and the class book to take home and study the sections (they give you a list of the sections/material on the test), then you proceed to come back in August (I think), and take the test. My school requires getting an 85% or better on the test ir order to pass. Next year I'm taking Physics and Chemistry along with Pre-Calc as the math course. The reason for this is because many people have recommended that Pre-Calc be taken alongside Physics. Also, the numerous textbooks that I've scammed through state that the required math skills are Alg II, Geometry, and some Pre-Calc, this is assuming that you will be taking the Alg II based Physics class. Which you probably are (unless is the straight AP one, not the Pre-AP), so in that case you won't be required to know all that much Pre-Calc, the basics at most, but if you decide to take the AP course instead of the regular or Pre-AP one then I would highly recommend gaining strong Pre-Calc skills over the summer, testing out, and take AP Calc your Junior year.
Hope this helped!