  An exact analytical solution of the ground state problem of the isotropic
classical Heisenberg model on the Shastry-Sutherland lattice in external
magnetic field $H$ is found for arbitrary ratio of diagonal to edge exchange
constants $J_2/J_1$. The phase diagram of this model in the ($J_2/J_1, H/J_1$)
plane is presented. It includes spin-flop, spin-flip and umbrella phases. The
magnetization curves are found to be linear until saturation. It is shown
numerically that the inclusion of the easy-axis anisotropy into the model leads
to the appearance of the 1/3 magnetization plateau, corresponding to the
collinear up-up-down spin structure. This explains the appearance of the 1/3
magnetization plateau in rare earth tetraborides RB$_4$. In particular,
magnetization curve of the compound HoB$_4$ is explained.
