More than half of those surveyed - 51 per cent - thought the "Biebs" was much cooler than Mr Key, who got 49 per cent of the votes.
Other questions included what the Duke and Duchess of Cambridge should have named their son.
The top name, with 39 per cent of votes, was Benji - possibly a nod to popular Kiwi rugby league star Benji Marshall. The second coolest name with youngsters was Sonny Bill.
The Cool Our Schools initiative offers a share of $500,000 worth of art and sports gear to primary and intermediate schools around the country. Students from dozens of primary and intermediate schools took part in the survey.
Meadow Fresh spokeswoman Laurie Piggott acknowledged that the results made for interesting reading.
"It was brilliant to see that kids think the prime minister is almost as cool as Justin Bieber - the votes were so close.
"We also thought it was great to see how many kids were interested in what the Duke and Duchess of Cambridge should have called their new baby. We loved the fact that they feel they should have named him after some of our best sports stars."
Other results indicated that if they were given a choice to pick a new milk flavour, Bubblegum would be the coolest flavour, with 49 per cent of children voting for it.
The coolest job, as voted by 40 per cent of voters, would be as a sportsperson. Being a teacher, builder or prime minister were not such cool jobs, each getting five per cent of the votes.