I live and work in Miami and would like to bring my niece to study in high school in the area where I live. It seems she would need an F1 visa and I wonder if there is any agency that could help me with all the legal aspects.
You've probably done much research into the visas for the US, and actually processing it can be confusing. You can find a relocation company to help and I advise you to contact your nearest embassy for the most updated info.
Have you looked for local relocation companies in your area? Expat Arrivals lists some major relocation firms that help with a whole range of relocation needs including immigration and visa processing – check our a list for Relocation Companies in Miami here.
Have you already got a school in mind for your niece? Expat Arrivals guide to your city includes some useful tips on Education and Schools in Miami that could be helpful to you.
All the best.