Good luck with your braces Pearl :o)
I had double braces with elastic bands between the top and bottom set when I was 12/13 and it made such a dramatic difference over about 18 months, now my teeth have remained straight through my adult life.
It will affect your speech slightly but only for a very short while as Cloverjo says, until you get used to it, but as to pitch and tone, no - your singing shouldn't be affected.
You might find that your mouth is a bit sore when trying to enunciate during your songs as your braces may catch slightly inside your mouth, but if this happens, just go back to the orthodontist and have your braces adjusted slightly and to ensure there are no rough bits.
The one thing I wish I did more when I had braces was to go overboard with cleaning my teeth. Food can get stuck so easily, so make sure you have a toothbrush with you at school to clean after lunch.
All the best :-)