I've just finished in my role and can remember the stress from the application process with all the speeches and interviews etc.
I remember being asked:
Why do you want the role of Head Girl?
What will you change whilst in the role?
How do you cope under pressure, and can you give an example of when you did so?
What qualities do you think are important for the role?
What are your strengths?
What are your weaknesses?
Don't panic and rush to answer the questions - don't rush to answer - and remember that they want you to do well as much as you do!
Good luck!