The insulating Sr1-xEuxS system exhibits spin-glass properties for x≳0.05 due to competing ferro and antiferromagnetic interactions. A scaling law x/Tf=f (ln ν) represents the frequency (ν) dependence of Tf, the temperature of the maximum of χa.c. (or spin-glass freezing temperature). This frequency dependence of Tf is at least an order of magnitude bigger than in a canonical spin-glass such as AuFe. The dipolar interaction is responsible for the anisotropy inside magnetic clusters. For x<0.05 the a.c. susceptibility, χa.c., doesn't exhibit a single maximum, but after an initial maximum or plateau around Tp it increases to a second rounded maximum, at much lower temperatures. Below Tp, which is concentration independent, a remanent magnetization appears proportional to x2. Furthermore, at this temperature deviations from a Curie law of χa.c. increase more rapidly than x. Moreover, the frequency dependence of Tp permits a determination of a blocking energy. This energy is in agreement with the anisotropy barrier due to the dipolar interaction for a ferromagnetic pair of first neighbors. All these results support the idea of short range order for individual pairs, and small magnetic clusters which are blocked below Tp on random anisotropy directions determined by the intra-cluster dipolar interaction. A similar change in the magnetic properties of other spin-glasses is expected.