Geulen taught and lived at the Athénée royal Isabelle Gatti de Gamond, a boarding school in the Brussels suburb of Woluwe-Saint-Pierre, whose headmistress, Odile Ovart-Henri [fr], had already agreed to host 12 hidden children at the school. The school on the Rue André Fauchille was raided by the German authorities on Pentecost in May 1943 and some of the children were detained. Geulen herself was interrogated, but escaped to warn some of the Jewish pupils who had not been at the school at the time. Ovart and her husband Remy were both deported to a concentration camp, and did not survive the war. In the aftermath of the raid, Geulen went into hiding with Sterno and assumed a false identity, as Claude Fournier. Sterno was arrested in May 1944. Geulen was personally involved in the arrangements made for 300 Jewish children whom she collected and accompanied to Catholic schools and monasteries. 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