Wellington College head Roger Moses has challenged Green MP Nandor Tanczos to meet principals to discuss cannabis law reform.

The occasion was a public debate between Mr Tanczos and Associate Health Minister Jim Anderton at the Johnsonville Community Centre on Wednesday evening that attracted 150 people.

"Day in, day out, we are faced with young people who are de-motivated and dropped out," Mr Moses said.

"Yes, your arguments are plausible, (but) I don't think they are credible." Mr Tanczos readily accepted Mr Moses' challenge.

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