I have.
I might not have chosen the best class to enter, but Edinburgh's music festival has lots of classes specifically for kids, and a very few with open age access: I actually contacted the organisers to ask advice, because I wanted experience of playing to an audience, with some feed-back, and this seemed a good way to go about getting that. For my level at the time, they suggested I try the Recorder Medal class, or maybe the concerto class - for the latter there'd have been the option of an orchestra if I'd got through to the final, but the problem would've been finding a pianist who could do the orchestral reduction for the first round, and that wasn't something I could muster.
So, I did the recorder medal. I found I was up against three teenagers, all taught by the same lady, and did rather get looks along the line of "this is for talented kids, what are you doing here?" The requirement was for a fifteen minute recital: they did two pieces each, one modern, one baroque......and I did four shorter ones including an avant garde one. At the end the adjudicator picked my performance rather more to bits than the others, before then saying she'd decided to award the medal to the one who was probably the most experienced - me. It left a nasty taste in my mouth, the more so as it wasn't actually true - I'd had half the years of lessons the kids had had.
The final twist came two days later, when I got a phonecall at eleven at night, telling me I'd been awarded a place in the grand finale concert for the festival as a whole....playing the avant garde piece, which I'd only put in because I'd read the biography of the adjudicator and realised she mainly plays that sort of
tripe er, material

.