Regardless of today's new law making psychoactive substances such as synthetic cannabis illegal, for those under 18 they were always so. As are all highs, not just classified drugs but nicotine and alcohol and so-called "natural" cannabis.

For some under-18s, today's change in law does not matter a fig, as they choose to indulge in their drug of choice seemingly regardless of the consequences.

As well as parents, schools play an important role in enforcing the law. On Monday, we reported how a German international student had been expelled from Tauranga Boys' College for smoking cannabis and faced being sent home.

The boy, 17, bought $80 worth of marijuana for himself and other German students to smoke in a park after school. He said he had one puff and drove home on his motorbike. After the school uncovered the story and spoke to him, his contract with it was terminated.

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However this has been labelled excessive by the boy's family, who say international students are held to different standards than locals. The family has engaged in a five-week legal stoush with the school, with the boy's uncle and designated caregiver in New Zealand arguing that he only had a puff as "they all did it", and that he should not have been expelled as it was not done at school.

Principal Robert Mangan is sticking to the hard line that the school has zero tolerance for illegal drug use.

It would have been easy to take a softer approach, particularly given international students are a cash cow for schools. They contributed $2.6 billion to the economy in the 2012-13 financial year.

Some may argue that because it was outside school hours it should be a police matter with no additional punishment by the school. I don't agree.

The boy knew the law. He rode home under the influence and was a potential hazard on the road.

As Secondary Principals' Association president Tom Parsons said: "He knew the consequences before taking that puff so welcome to the real world, junior."

Good on him and Mr Mangan for giving the boy a crucial lesson that he seemingly won't be receiving from his family.