In the framework of a (222)Rn screening campaign that was carried out in 58 public secondary schools in Galicia (NW Spain), the largest radon-prone area in the Iberian Peninsula, a positive correlation between indoor (222)Rn concentration and outdoor gamma exposure rate was obtained. A new approach to the data acquisition in screening surveys was tested, improving the performances of this type of study and gathering useful data for future remedial actions. Using short-period detectors (charcoal canisters) firstly, in order to detect places showing (222)Rn concentrations over 400Bqm(-3), the number of locations to be measured with long-period detectors (etched track detectors) is reduced. In this screening campaign, 34% of the schools surveyed presented at least one site exceeding the 400Bqm(-3) recommended action level established by the EU, and 15% had at least one site with (222)Rn values over 800Bqm(-3). The maximum value recorded was 2084+/-63Bqm(-3). These results are discussed and compared with data obtained in schools of several countries with similar geology. Seven schools were also studied for seasonal variations of (222)Rn activity concentration. The results were not conclusive, and no significant correlation between season and (222)Rn concentration was established. Finally, a continuous (222)Rn concentration monitor was placed in the secondary school exhibiting a mean value of the (222)Rn concentration very close to 400Bqm(-3). Maximum (222)Rn concentration values were found to occur at times when the school was unoccupied.