School principals nationwide are being asked to get on board with Australia's world-first social media ban as the government fends off criticism the policy is being drawn up on the fly.

A letter from federal ministers Jason Clare and Anika Wells has been sent to the leaders of all schools in Australia, urging them to promote and support the delivery of the ban for users younger than 16 when it starts on December 10.

While the ban won't completely shield young people from harmful content, the letter emphasises that schools are vital in continuing the conversation on preventing online harm.

"Schools and teachers are on the front line of helping young people deal with the online world," Mr Clare said on Saturday.

"We're giving them the support they need to help keep kids safe."

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