We prove that for every complex classical group $G$ the string polytope associated to a special reduced decomposition and any dominant integral weight $λ$ will be a lattice polytope if and only if the highest weight representation of the Lie algebra of $G$ with highest weight $λ$ integrates to a representation of $G$ itself. This affirms an earlier conjecture and shows that every partial flag variety of a complex classical group admits a flat projective degeneration to a Gorenstein Fano toric variety.