PARIS (REUTERS) - Brussels suicide bomber Najim Laachraoui, a veteran Islamist fighter in Syria also suspected of making explosive belts for November's Paris attacks, was a model student in a Brussels Catholic high school, the school's director told Reuters.

Security sources told local media that Laachraoui, a 25-year-old Belgian, was one of Tuesday's airport suicide bombers, identifying him as one of the three men in the CCTV image released by police.

"Najim Laachraoui was a very good student," said Veronica Pellegrini, the director of the Institut de la Sainte Famille d'Helmet, a Catholic school in the ethnically mixed east Brussels borough of Schaerbeek.

"He never failed a class," Pellegrini said of Laachraoui, who studied at the school for six years, until graduating in 2009.

"We haven't heard from him since," she told Reuters in a phone interview.

Prosecutors said Laachraoui's DNA was found in houses used by the Paris attackers last year. He left Brussels for Syria in February 2013. Local media has said he has technical training that could mean he was the armourer of the operation.

Travelling under the false name Soufiane Kayal, he was documented driving from Hungary into Austria in September in a car driven by Salah Abdeslam, the prime suspect in the Paris attacks who was arrested in Brussels last week.

There is speculation Laachraoui had just returned from Syria, possibly by sea with refugees.

Catholic religion classes are part of the school's curricula for all students independent of their religion, and Laachraoui would have attended those classes as any other student, Pellegrini said.

While the school offers technical studies in fields such as chemistry, and Belgian media say Laachraoui had studied electromechanics, Pellegrini said he did not take such courses in her school, where he only pursued general studies.

The school's motto is a verse from the Bible (John 3:14-18): "We know that we have passed from death to life, because we love each other... Let us not love in word or talk but in deed and in truth."

It is not uncommon for Muslim pupils in Belgium to go to Catholic schools, which can be seen as more conservative or more exclusive than state schools.

The Paris attacks' suspected mastermind, Abdelhamid Abaaoud, a Belgian killed during a raid in Paris suburb St Denis on Nov 18, had at 12 won a scholarship to an elite Catholic school.