In an April 27 email, the school’s director, Chris Pijpen, told an education official, Charles Huygens, that Mr. Hadfi had not attended school since Feb. 24. Officials met with his mother and aunt on March 23; they said he had left for Morocco, where he had relatives, because he was “fed up” with school.

But the school was rife with rumors that Mr. Hadfi had left for Syria, according to the email. It included images from Mr. Hadfi’s Facebook page, where he had adopted a nom de guerre, Abu Moudjahid al-Belgiki (the surname means “of Belgium”) and posed with a jihadist flag.

The email was reported by De Morgen, and then provided to The New York Times. Mr. Huygens did not alert the police. “It’s true that the director told me by mail in April that our student had departed to Syria, but by then it was already too late,” Mr. Huygens told De Morgen. Mr. Pijpen said he did not contact the authorities himself because protocol required that he go through his superiors.

Belgian law requires that schools report suspicions of terrorist activity to the authorities. In a phone interview on Saturday, Mr. Pijpen said he never got a reply to his email. “I expected that something would happen, some further action, at least someone from the administration that would come down to our school, or the police,” he said. “I was amazed that nothing happened. This was already after Charlie Hebdo and Verviers,” a Belgian town where the police in January killed two men suspected of links to a terrorist network plotting an attack.

“You would expect some reaction,” he added. “But then again, we’ve been asking for years for more support at our school, or the hiring of specialists, but never got any.”