When he was just 13, he was so conscious of the perils of being a teenager that he helped set up a website offering support to disaffected teens.

Four years later, authorities believe the high-achieving Sydney Boys High student had himself grown increasingly disaffected.

The 17-year-old, who Fairfax Media has chosen not to name, and his brother, 16, were stopped at Sydney Airport on March 6, believed to be on their way to the Middle East, via Kuala Lumpur, to join Islamic State.

In a concerning development for counter-terrorism authorities, it appears the pair were the antithesis of the typical teenager being lured to Syria and Iraq by the Islamic State.

They did not appear to be marginalised, lonely, unhappy or bored.

Fairfax Media can reveal that authorities are looking into the three months the older brother spent in Germany over the summer holidays, examining whether he came into contact with any extremists who may have prompted his sudden and rapid radicalisation.

He was one of two students to win a scholarship to live with a German family for 10 weeks, departing in November 2014.

Less than a fortnight after he returned home, he was allegedly intercepted at Sydney Airport with a return ticket to Turkey.

The star Year 11 pupil at the prestigious GPS school was a regular academic prize winner, a member of the debate team, a state-level football player and recently voted in as vice president of the Student Representative Council by his peers.

Among the topics covered at inter-school debates one year was whether the West "should undertake surgical air strikes against Assad's regime in Syria".

In 2011, he urged his friends to get behind an internal school website providing help for "getting through adolescence".

"This site is extremely beneficial to the problems that young adolescents could be having out there. Let's try harder to make the site better," he posted.

Just weeks ago, he won a prize for his outstanding score in an Australian Mathematics Trust competition.

His younger brother was also an avid sportsman, excelling at basketball, long jump and football.

"If they were switched on to questions of global injustice and seeking answers, not just angry young men or alienated young men but passionate idealists, then they can just as easily be targets of the predatory recruitment by Islamic State," said terrorism expert Greg Barton, from Monash University's Global Terrorism Research Centre.

"It is deliberate, targeted, predatory recruitment."

Immigration Minister Peter Dutton said the boys' parents, who live in south-west Sydney, had no idea of the boys' attempt to travel to "a potentially very dangerous situation".

The younger boy, a Year 10 student, wore a yellow 'Australia' T-shirt to the airport and the pair had allegedly deleted electronic files with tips on how to convince Customs officials they were tourists.

"As a result of this interception, a Sydney family remains together," Mr Dutton said on March 8.

The Australian Federal Police released the boys into the care of their parents without being charged. A spokesman said an investigation is ongoing.

The younger child had watched a video recently by Sheikh Feiz Mohammed, a reclusive, firebrand who has been blamed for radicalising Sydney men including wanted terrorist Mohamed Elomar.

The eldest brother had recently attended some seminars by a University of New South Wales Islamic group.

Sydney Boys High principal, Dr Kim Jaggar, would not comment on the case.

A spokesman for the Department of Education said they assist investigative agencies in such matters and always place the highest priority on student wellbeing and welfare.