in my gym our trainer puts the girls against the guys. i don't want to sound sexist, but usually the guys are better so it would be advantageous to box an experienced guy that's around your age and stats.
Get a lot of protein in your body as well as fibre, once you get the clay you can start to sculpt it, know what I mean?
I'm 13 114 pounds 5.2. My muscles are pretty good for my age, I just eating a lot of protein and working out and getting lots of healthy food helped me from being so petite, I'm about average now, not underweight.
I am not sure there's a weight division for such a small weight though. (If you want to fight) Get about 5 pounds lower then the least allowed and tone your body out.