Carley Wenham and Charlotte Browning proved themselves against a world class field three years their senior.
Carley was sixth in the 200m as was Charlotte in the 1,500m at the World Youth Games at Sherbrooke, near Montreal, Canada.
The pair, who missed the All England Schools Championships to compete, gave away three years in a competition for under-18s.
Carley, 15, from Crawley, clocked 24.11sec into a headwind in the final after breaking her own Sussex senior record with 23.59sec, the third fastest time by any British woman this season, in the heats.
She said: "It was the most amazing athletics experience of my life. I was against all the top Americans, who get fantastic sponsorship, and I proved I could hold my own.
"It was the first time I'd been abroad and also the first time I'd gone away without my parents. That was a little scary.
"A part of me missed the All England as everyone gets together for Sussex but the other part knew I had to go to Canada because it took my career on."
Charlotte, 15, from Chichester, ran a personal best 4min.22.16sec in her final after qualifying as a fastest loser.
Browning, a multi-national champion, said: "It goes straight to the top of my athletics experiences."
Carley and Charlotte's times in Canada would have ensured national titles for each of them at Sheffield.
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