Polarization operator for 2+1 Dirac fermions is evaluated at finite temperatures. Reflection coefficients of an electromagnetic wave from a plane fermion layer are derived. Theory of the Casimir effect for interaction of a plane layer of 2+1 Dirac fermions and another material with a flat surface separated by a vacuum slit is presented. The high-temperature behavior in a graphene-metal system starts at separations between graphene and metal of the order of 100 nm at temperature T = 300K. The high-temperature asymptotics of a free energy for graphene-metal, graphene-graphene systems coincides with the high-temperature asymptotics for two Drude metals separated by a vacuum slit.