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The state government announced the move on Thursday, following a review into their use in classrooms.
"Mobile phones, unfortunately, are not only distracting but also causing stress for young children - and we can't have that continue," NSW Premier Gladys Berejiklian told Seven's Sunrise.
"We're sending a very strong message to everybody that children need to be protected."
The education department will provide guidelines to schools about exactly how the ban will work, but it's believed students won't be able to have access to phones during the day.
If there are special requests from parents they will be kept somewhere to be accessed before or after school, Ms Berejiklian said.
High schools will have the choice to opt-in to the ban.
Psychologist Michael Carr-Gregg, who led the review, said mobile phones posed a risk to children.
"It's all really about kids focusing on lessons, better socialising, reducing social media use and reducing online bullying and online image abuse," he told 2GB.