  In a graphene {\em pn} junction at high magnetic field, unidirectional "snake
states" are formed at the {\em pn} interface. In a clean {\em pn} junction,
each snake state exists in one of the valleys of the graphene band structure,
and the conductance of the junction as a whole is determined by microscopic
details of the coupling between the snake states at the {\em pn} interface and
quantum Hall edge states at the sample boundaries [Tworzydlo {\em et al.},
Phys. Rev. B {\bf 76}, 035411 (2007)]. Disorder mixes and couples the snake
states. We here report a calculation of the full conductance distribution in
the crossover between the clean limit and the strong disorder limit, in which
the conductance distribution is given by random matrix theory [Abanin and
Levitov, Science {\bf 317}, 641 (2007)]. Our calculation involves an exact
solution of the relevant scaling equation for the scattering matrix, and the
results are formulated in terms of parameters describing the microscopic
disorder potential in bulk graphene.
