For juniors, the biggest advantages of boarding are 1.) riding in a supervised situation, rather than by yourself or with a parent observing 2.) other people/kids to ride with 3.) easy access to training and instruction 4.) facilities -- rings, jumps, etc.
Me? I keep my horses on my own property and love it; and prior to that leased property to keep them. Wild Spot gave a pretty good list of the disadvantages, I've encountered all of those things in my boarding experiences. However, the social and safety aspects of boarding make for pretty powerful arguments in your situation.