On the Justice website, under the FAQ section, this is exactly what they say:
How can I become a Justice or Brothers model?
Justice and Brothers do not currently have their own models. Depending on the photography requirements, we usually use modeling and talent agencies that represent young people. Justice and Brothers do not offer agency referrals. You may want to contact such an agency in your local area to see what is involved in becoming a model.
In other words, any teen girl's best chances of becoming an actual Justice model is to get signed to a modeling agency that currently has Justice as one of its clients. Why? Because it shows that the agency has already provided Justice with models in the past, which increases the odds that the company will turn to them again for future model castings. Additionally, it also shows that the agency works with quality, high end companies and brands.
They don't state the names of the agencies they use, nor do they refer aspiring models to them. Remember, folks, Justice is a company and is all about putting out their products and making a profit. They don't have the knowledge, resources, time or interest in helping people start their modeling careers--it's simply not their job.
However, through their website I did see that Justice's locations (their website headquarters and customer relations department) is based out of Ohio. So there could be a chance that modeling agencies in the Ohio area are the first ones that Justice turns to. I don't know that for sure but it's just a hunch, since most retailers will use local agencies/models since it's convenient.
If you happen to live in Ohio, it's worth checking out the agencies locally to see if any of them have worked with Justice before.