Celladore,
I recommend that you ignore your teacher's advice. Your passion for the subject shines through on this forum, and I think that will carry you.
I had always been quite vocal in lessons at school, and carried this through in my degree studies. But this is by no means a requirement. First, there are people on my course who came to the uni being rather shy to speak their mind in front of others, but law school will, over the three years, turn you into a confident graduate. This will arguably be more so than other subjects. Secondly, you ought to realise that your degree is assessed through what you write, and has no advocacy component. It is perfectly possible to do well in your studies without being vocal, so it is by no means a requirement.
If I was in your position I would not be deterred from applying. Hell, I would walk right up to her and tell her exactly why she is wrong; perhaps that would prove to her that you have the cajones for the course. If you just sit back and tdo what she recommends then you will only risk proving her right...