Jackson won her first All Schools state cross-country, taking out the under 17 girls' race in 16.20 minutes ahead of Prospect High's Zoe Casey and Burnie High's Isabel Gitzinger.

The teenager has come on in leaps and bounds in her endurance thanks to her triathlon training.

"Last year I came 16th, so to come first this year, it's a big achievement for me. I'm really happy," the 16-year-old said.

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The under-17 and under-18 girls started together and Jackson explained how the four-kilometre race panned out.

"The two older girls went out first and I kept with them until about 500m and then they went off ahead," she said.

"I just kept going at my own pace, trying to keep catching the next person and all the boys in front of me as well."

Jackson, who is part of the Launceston Triathlon Club, did her first tri when she was six and has taken the sport more seriously in the past two years.

Ironically, running isn't even her favourite leg and she's a keen cyclist.

"I've only been doing competitive running for the last year and a half. Mum and dad are both runners and so is my brother," she said.

"That helps having them to help me train."

A bit of sibling rivalry helped spur her on during the race on Tuesday.

She said her younger brother Harry, who is in grade five, came second in his All Schools event last week.

"It's good to have some competition against him to push myself a bit further," she said.

Jackson will represent Tasmania in triathlon in Hervey Bay in September. She explained how she qualified for that All Schools event.

"I've been doing heaps of local ones but I had to put in an application with all my events and achievements in all three disciplines," she said.

Meanwhile, Scotch Oakburn College's Sophie Marshall continued her dominance at All Schools taking out the under-16 girls' event in 15.07 minutes.

She has won five times at All Schools across primary and secondary school.

The teenager described the hilly, windy and slippery conditions as tough to navigate.

But she enjoyed continuing the friendly rivalry she has with St Mary's Jessica Smith who finished second.

They kicked away in the second lap with the field well spread out by the end.

Marshall, who also competes for Newstead Athletics, explained she had been dealing with a long-term foot injury.

It meant the talented 1500 and 3000m runner, who is coached by Liz Gray, couldn't go to nationals in Sydney this year.

The youngster said it was good to have a break but also hard to deal with a lingering injury.

Marshall's excited about competing at the national titles in Adelaide.

She said her best nationals cross-country result was coming fifth.

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