We present a study of the magnetic properties of the electron doped manganites Ca_1-xY_xMnO₃ (for 0 ≤ x ≤ 0.25) in the paramagnetic regime. For the less doped samples (x ≤ 0.1) the magnetic susceptibility, χ(T), follows a Curie-Weiss (CW) law only for T > 450 K and, below this temperature, χ^-1(T) shows a ferrimagnetic-like curvature. We approached the discussion of these results in terms of a simple mean-field model where double exchange, approximated by a ferromagnetic Heisenberg-like interaction between Mn³⁺ and Mn⁴⁺ ions, competes with classical superexchange. For higher levels of doping (x ≤ 0.15), the CW behaviour is observed down to the magnetic ordering temperature (T_mo) and a better description of χ(T) was obtained by assuming full delocalization of the e_g electrons. In order to explore the degree of delocalization as a function of T and x, we analysed the problem through Monte Carlo simulations. Within this picture we found that at high T the electrons doped are completely delocalized but, when T_mo is approached, they form magnetic polarons of large spin that cause the observed curvature in χ^-1(T) for x ≤ 0.1.