That would depend on the way the groups are drawn. The ancestors of mammals are classified as synapsids, rather than reptiles.
Introduction to the Synapsida"These pre-mammalian groups of synapsids have at times been called "mammal-like reptiles". This term is now discouraged because although many had characteristics in common with mammals, none of them were actually reptiles"
However, "fish" seems to be a term used for the "basal vertebrates" (same site) so, on that basis humans could be classified as fish .
This site is also worth a look:
Tree
of Life Web Project