  We perform an extensive test of theoretical stellar models for main-sequence
stars in ugriz, using cluster fiducial sequences obtained in the previous paper
of this series. We generate a set of isochrones using the Yale Rotating
Evolutionary Code (YREC) with updated input physics, and derive magnitudes and
colors in ugriz from MARCS model atmospheres. These models match cluster main
sequences over a wide range of metallicity within the errors of the adopted
cluster parameters. However, we find a large discrepancy of model colors at the
lower main sequence (Teff < ~4500 K) for clusters at and above solar
metallicity. We also reach similar conclusions using the theoretical isochrones
of Girardi et al. and Dotter et al., but our new models are generally in better
agreement with the data. Using our theoretical isochrones, we also derive
main-sequence fitting distances and turn-off ages for five key globular
clusters, and demonstrate the ability to derive these quantities from
photometric data in the Sloan Digital Sky Survey. In particular, we exploit
multiple color indices (g - r, g - i, and g - z) in the parameter estimation,
which allows us to evaluate internal systematic errors. Our distance estimates,
with an error of sigma(m - M) = 0.03-0.11 mag for individual clusters, are
consistent with Hipparcos-based subdwarf fitting distances derived in the
Johnson-Cousins or Stromgren photometric systems.

Solar and Stellar Astrophysics;Astrophysics of Galaxies