We present results of a Monte Carlo study over the ferromagnetism of Co-doped ZnO. The magnetic interaction has the form of the donor impurity band exchange model, where the Co magnetic moments are exchange coupled to band electrons. These are assumed to occupy large hydrogenic orbitals and originate from shallow intrinsic ZnO defects. A number of parameters of this model remain uncertain and here we investigate the dependence of the Curie temperature on the strength of the magnetic coupling. We find an unusual concave upward shape in the magnetization curves consistent with other Monte Carlo studies for dilute systems and we predict high temperature ferromagnetism for sufficiently strong coupling.