Dekker took victory in the 15 year-old girls’ 100m freestyle competition at Ponds Forge.
She clocked 57.89 seconds to claim the title in a race that saw her Mount Kelly team-mate Georgina Dennis come fifth and Plymouth’s Duru Tanir sixth.
Leander’s Jackson (pictured), who will represent Great Britain at next month’s World University Games in Chinese Taipei, took bronze in the senior women’s 100m freestyle.
The Plymouth University student clocked 55.47 seconds to finish behind Sheffield’s Ellie Faulkner and Edinburgh’s Lucy Hope.
Mount Kelly’s Athena Clayson just missed out on a podium placing in the 16 year-olds’ 100m freestyle. She finished fourth in 57.60 seconds.
There were also three fourth places for Leander on Friday evening in South Yorkshire.
Salvador Gordo was fourth in the 13/14 year-old boys’ 100m butterfly in 59.49 seconds, as was Honey Osrin in the 13/14 girls’ 200m backstroke, while Leander also finished just outside the medals in the boys’ 14-16 year-old 4x200m relay event.
There were also final appearances for Leander’s David Miller, Anita M’Cartney, Reid Jones and James Watson, while Mount Kelly’s Andrew Paige and Lily MacLeod also made finals.