The general requirements for being a heroic Spirit is to be "An object of worship or revered" and "To have a soul". 

The 'School of Muramasa' has these requirements and the fact that They are attributed to a single entity more than individuals... Its possible for all who meet the criteria to be condensed to try and fit this 'Image' that humanity has. The Throne of Heroes is removed from time and space... So it could have recorded the entire school as an Anti-hero, or an entity that's a cautionary existence. Especially since All swords forged by Muramasa were outlawed, and the line between school and individual has become abhorrently blurred. If asked most people would think of Muramasa as an individual instead of a school... 

Again its up to the GM, but I argue its within the realm of possibilities. As I said this is a 'False' image not the genuine article.