Why not just call them grades? Mountain Scholars Latin Heritage School seeks to meet students right where they are, and an over emphasis on the construct of grades is not helpful in meeting that goal. All of our class levels are made up of are multi-age students who are learning best as they are placed in the level that appropriate to their needs.
In addition to eschewing traditional grade levels nomenclature, on campus we call our classes by their mascots–each is named for a different Talking Beasts from Narnia, and is assigned an accompanying virtue.