By performing pump-probe experiments, we study the relaxation dynamics of spin-polarized excitons in a wurtzite GaN/AlGaN quantum well (QW)grown by metal organic vapor phase epitaxy. The circular degree of polarization of the differential reflectivity signal reaches 55-60 % for the heavy-hole exciton when it is resonantly excited before decaying in a few picoseconds. This is an indication that, even if excitons are confined in the growth direction, their propagation in the well plane makes them very sensitive to scattering processes which are responsible for exciton dephasing and, therefore, for efficient spin relaxation. Above 80 K the thermal escape of excitons toward the AlGaN barrier increases the spin-relaxation rate. However, when compared to bulk GaN epilayers, where no spin polarization could be detected above 50 K, the optical orientation is preserved in those QWs up to about 100 K.