Muxlow has already won a British judo title this year and her school, Hill House at Doncaster, entered her for the Doncaster schools event.
The Year Nine pupil was competing in the under-15 javelin and was up against the best in the region.
But after throwing 29.23m, which was only just short of the England schools qualifying distance for her age, to win by four metres.
At the Hill House sports awards, she was named the 2013-14 player of the season for both netball and rounders.
But she improved on that a week later when she was honoured to receive the outstanding sports shield presented by Captain James Shelton, Adjutant 45 Commando Royal Marines at Hill House School Senior School speech day and prize giving.
To complete an excellent few weeks, Muxlow won the javelin and discus at her annual school sports day.