What a great question! As with many professions, you will take a range of classes at different levels during your training, already starting in high school. Success in your career does not depend on any one class or theme, and some of the greatest thinkers have needed time to mentally digest concepts in a different way than others in order to make their greatest discoveries. So, please don't let lack of immediate success in one subject hold you back from pursuing your dreams. At the same time, get as much experience as possible in math and science and engineering, as well as in writing and speaking in English and other languages, since global communication is vitally important.
Another thing to consider is that you will continue to develop personally and professionally throughout your teens and twenties, and thirties - indeed, throughout your life! All of the subjects with which you come into contact along the way, whether school subjects or life experiences, will come to bear at one time or another. As they say, we learn more from our failures than our successes, so do your best and pursue your dreams! Keep taking the hardest classes you can, pushing yourself out of your comfort zone, even if it means getting a lower grade. In this way, you maximise your potential for growth at any given stage in your life.
Finally, try not to worry about not being 'good enough'. Usually those who worry most about this should worry least and vice versa.
Biomedical engineering is a great field because it brings so many different subjects and human experiences together. It's incredibly interesting, fun and impactful!