  We construct a supersymmetric standard model in the context of the $Z_{12-I}$
orbifold compactification of the heterotic string theory. The gauge group is
$SU(3)_c\times SU(2)_L\times U(1)_Y\times U(1)^4\times[SO(10)\times U(1)^3]'$.
We obtain three chiral families, $3\times\{Q,d^c,u^c,L,e^c,\nu^c\}$, and Higgs
doublets. There are numerous neutral singlets many of which can have VEVs so
that low energy phenomenology on Yukawa couplings can be satisfied. In one
assignment (Model E) of the electroweak hypercharge, we obtain the string scale
value of $sin^2\theta_W^0={3/8}$ and another exactly massless {\it exphoton}
(in addition to the photon) coupling to exotic particles only. There are color
triplet and anti-triplet exotics, $\alpha$ and $\bar{\alpha}$, $SU(2)_L$
doublet exotics, $\delta$ and $\bar{\delta}$, and $SU(3)_c\times SU(2)_L$
singlet but $Y={2/3},-{1/3},-{2/3},{1/3}$ exotics, $\xi,\eta,\bar{\xi},
\bar{\eta}$. We show that all these vector-like exotics achieve heavy masses by
appropriate VEVs of neutral singlets. One can find an effective R-parity
between light (electroweak scale) particles so that proton and the LSP can live
sufficiently long. In another assignment (Model S) of the electroweak
hypercharge, there does not appear any exotic particle but
$sin^2\theta_W^0={3/14}$.
