One of the things I suggest to students who come and see me is to practice Amy Cuddy's power poses. If you have never heard of this, it is an amazingly simple tip, which anyone can do, and it only takes a couple of minutes:
This is the link to a short video describing her research into body posture and confidence at Harvard:
http://content.time.com/time/specials/packages/article/0,28804,2091589_2092033_2109441,00.html
What is particularly interesting, is that while she originally began investigating body posture in relation to students and achievement, the research results have helped people in all sorts of situations: speaking to their doctor, with stage fright, in job interviews. The list goes on and on. AND I think what you will find is there are activities such as yoga, which has wide stance, open poses, such as warrior pose, which are also useful.
The other activity that I really recommend is mindfulness. I know a lot of people are rabbiting on about this at the moment, but there is a reason - it works. What happens we have a feeling or thought that is uncomfortable is we try to avoid or deny the thought or feeling. Unfortunately, that actually makes it louder and more prominent. So mindfulness asks you to do something that becomes easier with practice: notice the thought or feeling and allow it to exist, but don't engage with it.
Here is an excellent talk by Ruby Wax about this: https://www.youtube.com/watch?v=x_DgOoKrkDA
She describes being able to view your thoughts and feelings as something outside of yourself - as if you are watching them on a television screen. Mindfulness takes practice, but it doesn't have to be hours and hours. A few minutes a day can build your skill.
And I also want to say that it is really normal to feel nervous about talking to teachers or anyone in a position of authority. The good thing is you can learn to be less nervous about it.