Nice.
Make sure the underarms aren't too tight or it will interfere with your sprinting.
There were a range of outfits on display at our (very ordinary state primary) sports day last week, from 'couldn't give a toss' right through to some carefull planned ones....but the scary teachers ensured that nearly all the parents in attendance did either the mummies or daddies race. Worth thinking about!
(I tried to hedge my bets in kne length cargo shorts with a mango floral tee and flat sandals. Bitterly regretted not slipping my trainers in my bag when I had to race barefoot and came second. Second!! With the right footwear I couldabeenacontender.