  The recent conjecture of possible equivalence between the string scale $M_S$
and the minimal supersymmetric standard model unification scale M_U ~ 2.5 x
10^{16} GeV is considered in the context of string models. This conjecture
suggests that the observable gauge group just below the string scale should be
SU(3)_C x SU(2)_L x U(1)_Y and that the SU(3)_C x SU(2)_L x U(1)_Y-charged
spectrum of the observable sector should consist solely of the MSSM spectrum.
We demonstrate that string models can actually be constructed that possess
these observable features. Two aspects generic to many classes of three family
SU(3)_C x SU(2)_L x U(1)_Y string models are both an extra local anomalous
U(1)_A and numerous (often fractionally charged) exotic particles beyond the
MSSM. Thus, for these classes, the key to obtaining an M_S = M_U ~ 2.5 x
10^{16} GeV string model is the existence of F- and D-flat directions that near
the string scale can simultaneously break the anomalous U(1) and give mass to
all exotic SM-charged observable particles, decoupling them from the low energy
spectrum. In this letter we show, in the context of free fermionic strings,
that string models with flat directions possessing these features do exist. We
present one such string derived model in which all such exotic observable
states beyond the MSSM receive mass at the scale generated by the Fayet-
Iliopoulos term. The associated F- and D-flat direction is proven flat to all
orders of the superpotential.
