It's possible you can just go against your teacher's wishes and do what you feel would be most appropriate, but before you do -
Have you told why you want a vocal piece by this justification? It just sounds like your teacher doesn't want a gazillion students doing voice, because of the image and the idea of being a pop-star - every other music class in other schools are filled with singers, believe me.
Also, he/she may feel that its severely competitive for the HSC. All you are asking is a reason. Ask for what alternatives you could use if he/she truly feels vocals are inappropriate - you just wanna achieve good results, and you're asking for advice.
If this doesn't work, you can discuss this with your principal, deputy principal or head teacher. Do not feel like you're dobbing them, just merely a second opinion. "Do you think its a good idea if i do vocal etc" and if yes, later tell them that you are not giving enough support or material to help you and was wondering if you could discuss your options. That way - you're not dobbing the teacher in - you're saying you're having trouble with the course and how to tackle the hsc in your current situations. Letting someone know would be a good idea.
That way, ignoring your teacher's wishes will be safer when the principal etc knows that you are concerned with your studies, rather than giving attitude to do the teacher.
You'll also find that musicals are hard to get together too , unless your school is predominantly filled with musicians...
Hope that helps.