Hi, it does depend which teeth are missing, and if by missing, are they stuck in the bone, or just not developed at all?
Lets assume that the teeth are not developed at all, and there are spaces where the teeth should be, your challenge is to get a nice cosmetic appearance, which fits in with her lifestyle, knowing that this may need to be changed in 7-10 years. (I normally advised waiting until you are about 21 before going for implants - if at all possible).
The most non invasive option for now is to have a removable plastic denture made. She may not love this, but it is definitely the quickest and simplest thing to do.
If the tooth position is not ideal, or the spaces need to be enlarged, then have braces now, and either a denture, or a temporary bridge.
Having a temporary bridge, this will look nice, the teeth will not be removable, and there is no tooth adjustment needed. To find out more about these temporary bridges, do a google search for sticktech, and find the videos on their site, these are designed for dentists, but show the idea really well.
I would avoid any permanent bridges at this stage, as her jaw and body are still developing.
I hope this helps a little!
Kind regards
Gurs