The tournament, held in Auckland, was contested by some of the top rugby Sevens-playing countries.
Cook Islands lost to Australia 38-7, New Zealand Maori 17-0, Fiji 34-7 and Tonga 25-17.
Cook Islands Rugby Union chief executive officer Ben Koteka said the tournament provided the team and the players with much-needed exposure.
More time together and tighter game protocols would see the team improve tenfold, he added.
Next for the U18s is a recruitment and retention drive for April 2018's Youth Olympics qualifier.
“A big meitaki maata to the team of young up-and-coming players, coaches Shane and travel manager Opura Samuel, Robbie Spear Atatoa, physio Gareth and their support crew and parents who provided meals during the camp.
“Thank you all for representing our proud nation at the World Sevens.”