Great Britain were the standout nation across all three days of competition in Madeira which featured 15 federations and around 500 competitors.
A biathle race is a continuous run-swim-run.
Highfield was first home ahead of Elizabeth Ryan and Francesca Middleton as GB claimed a clean sweep of medals in the under 13 girls competition, which saw athletes complete a 400m run, a 50m open water swim and another 400m metre run.
Great Britain dominated the race, securing the gold medal for the team competition.
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Greta, who won a bronze medal at the under 11 World Championships in Egypt last year, had moved up a level to the under 13 category and was competing against athletes a year older than herself.
Fellow Hill House pupil Olivia Martin finished sixth in the under 15 girls’ race after qualifying from the semi-finals the previous day.
Both girls are now preparing for the British Championships at Salford Quays and hope to be selected for Great Britain to compete at the World Championships in Florida, USA, in October.
Jonathan Lawrie, Swim Club coordinator at Hill House School, said: “Greta has had an outstanding year and it is no surprise that there is record interest in the school’s swim club from younger pupils who have seen what she has done.
“Olivia continues to compete with the best and although she just missed out on a medal she has a reputation for being one of the strongest swimmers in the Great Britain squad.”