A bit also depends on what grade you are. If you are near the end of your high school career and have taken physics, those lectures should be on par with your knowledge. If you are a freshman, you are probably ahead of the curve. The introductory lectures are exactly that, introductory.
Moreover, in our system (as well as yours, perhaps?) physics is not mandatory. Thus, introductory physics courses do not assume any background in physics. Most don't assume any knowledge of calculus either. They just teach you the chain rule and don't do anything more complicated.
In our system, a typical high school student who has taken physics should know the stuff in the Lewin lectures. A lot of students (especially at MIT) would have had high school courses far more rigorous. But not every student has had physics. In our system, if you have had physics in high school, you can test out of the introductory lectures. And at many schools they offer an algebra based course, a calculus based course, and an honors course (aimed at physics majors with a VERY strong background).
Hope that helps!