The truth is while the members of the school can join the River Academy company, there is little reason i see that the classes have to actually be held down in RiverBank. A central location would make it easier for people to get there and teach, especially from the North. As Guurzak mentioned, Rathglen is a rather ideal spot. It means new players need not travel as far to reach the Academy once Marchmont becomes the auto start point, its a neutral town that allows all the services of trainers, has access to an AH that will allow players to bring starting gear for new players to buy, and its still rather centrally located of all the NPC towns. With training being tied to the company/settlement you belong to not the settlement you do the training at, it does allow that members can join the academy company and still train there.
Granted, i understand this method does just turn RiverBank into a beacon to access training that does not need people to actually be there. But once more settlement options open up, i can see a reason to move the Academy back to RiverBank so classes and schooling can be dedicated to dealing with mechanics involved around how settlements work, but a lot of those mechanics are not yet implemented. Also a plus to having classes held at riverbank is there are towers there that RiverBank owns that can be used for training and classes involved with PvP in both attacking and defending tower sense, where up at Rathglen the school would need to use someone else's tower (unless a company/settlement was willing to offer a tower up for the school's use)
Regardless, i intend to help out with the school how i can, but i know a lot of new players are staying near their starting settlements, and RiverBank's distance far from starting towns does not mean players will actually be willing to make that trek to even attend the academy.