Whenever I send my kid to their first examination, I check their clothes.
Anyone under the age of 9 does not have the social skills to know what 'conservative casual' or 'casual formal' means. Up until this point, they wore 'clothes that are good enough for school and visits' and 'clothes that aren't good enough for school and visits' and 'jammies/swimsuits'.
The rule I give my kids: anything that isn't a T-shirt; anything that isn't jeans. Yes, there are loopholes, but I'm dead serious when I say this and they're too nervous to screw around.
If I'm lucky, the kid already has built in clothes. One of my students attends a Catholic school and wasn't too sure what to wear, but when I saw her school uniform (white polo shirt, black yoga pants) I gave it a pass. Her mother was anxious that it wasn't formal enough, but at that age examiners are pleased that you're not showing up in a miniskirt and flip flops.
(I've heard stories. IT HAS BEEN DONE.)
Yesterday I taught one of my kids in a last-minute lesson (her request). The kids typically wear pyjamas all day when they don't have to go out. At the end of the lesson I asked her what clothes she was wearing, and she wasn't too sure.
The upside of teaching at kids' homes: you can make them go and change into their clothes, RIGHT NOW.
She came back down with a nice button-up purple shirt and short jeans. I asked if she had pants that weren't jeans, she hesitantly said black yoga pants, I sent her back upstairs. Much better when she came back.
I then checked her shoes. She only had two: brown suede sandals with punched metal and fringes, and bright purple and white running shoes. At this point her grandmother came down to pay me and asked how the kid was doing. Through the kid (I understand Cantonese/Mandarin, but can't speak it very well; I do great pantomimes, though) I explained that the kid's clothes were okay...but she had no shoes.
Grandma: "Those sandals...no, where are your other shoes?"
Me: *pulls out purple shoes*
Grandma and me, in stereo: "Uh...."
They told me they'd go shoe shopping that evening. I had to tell them what kind of shoes to get (basically, the same shoes my kid's brothers had, but she currently didn't own...)
I like to think that I'm not just teaching piano, but teaching kids the basics of First World life that enable them to succeed.
* Picking your own exam songs: long-term planning
* Practising piano: discipline
* Dressing well: knowing what levels of clothing formality are needed
* Examinations and competitions: performing to an audience
I also consider myself a bit meaner than most teachers: while I give lots of positive encouragement, I let them know I expect only the best out of them. When we do practice marking, I'm taking away marks left and right because they should know better and I prefer to prepare them for the worst.
(I blame high school - when people reported that university marks dropped by 15 points (the freshman 15) because of poor high school discipline/inflated marking, none of my high school friends suffered that.)
The kids get nervous when I send them to competitions and examinations, especially for the first time, but they see how easy it is, or how other kids do in comparison to them. The first kid I sent came home from competition and started practising his song again, after seeing how four other kids did it.