The conservation of modern architectural heritage in seismic contexts must consider the analysis of more complex pathological processes than those traditionally associated with reinforced concrete deterioration. The behaviour of 21 school buildings built in Chile between 1937 and 1965 was comparatively analysed using the pathological diagnosis method. The documentation, in-situ observation, and measurements with non-destructive testing show that mechanical damages are more severe than physical-chemical damages. The levels and classification of deterioration are affected by accumulated damage due to the property's age and the characteristics of the architectural and structural design, which evolve from models of resistance by rigidity to flexible behaviour, which have lower levels of damages. In turn, the physical-chemical damage in the envelopes will depend mainly on the construction quality of the enclosures and the maintenance processes, rather than on the variation of the period's designs. An adequate pathological diagnosis of this architectural heritage allows the generation of appropriate conservation action guidelines to prolong the durability of these heritage works.