She started athletics from Grade-3 and won 50m and 75m races at the Primary Sports Meet in 2012 and 2013.
In 2014, she was selected to represent the school at the Sir John Tarbat Junior Athletics Meet and received a merit award for 100m and 200m races in the Under-12 category.
In 2015, she came third in 100m at the District Meet and in the following year, she came first in the Under-16 age category at the Sir John Tarbat Junior Athletics Meet.
After her outstanding performances, she was selected to represent the school relay team, which came first in the 4x100m event at the Sir John Tarbat Junior Championship.
Sithumi came first in the 100m race at the Kandy District Meet in 2016 and the schools relay team came first in the 4x100m event at the Zonal Meet.
In 2017, she was placed third in the 100m race at the Sir John Tarbat Meet and in 2018 she was placed first at the Zonal Meet.
She was selected as the Games’ Captain in 2021 and she was awarded School Colours for six consecutive years.