BRAINE-L'ALLEUD, Belgium—What could drive a 16-year-old boy from an affluent home in Belgium to leave behind his doting family, a prestigious private high school and the captaincy of his soccer team, in favor of a boarding school in a faraway war zone?For Raphael, a Jewish teen from the Brussels area, the answer was more than two years of antisemitic threats and intimidation, according to him and his family.In January, he left his parents, his 7…