Some children are just much more visible than others, they are in teams, do well academically, in school plays and so on. Mine are like that, it's partly because they are very self-confident and good readers so are able to learn lines/be relied upon to perform, but it's also partly because we do lots of activities, so my eldest gets picked for the swimming team each year not because she's a brilliant swimmer but because she has been in lessons for years and puts in a lot of effort. They join anything, will participate in everything, choir, plays, teams, after-school sports clubs, so it is not surprising they then appear in teams/plays (not always picked, not ever in a starring role, but just taking part).
I agree the school has to provide opportunities for all talents to be recognised and they do- there are no academic prizes at all (much to my husband's disgust), they do talent shows anyone can enter, anyone can be in the school play, in every class assembly all children speak and so on. But the plain fact is that some children are just good at everything, whether it be because they have a positive attitude and will have a go at anything, or because their family join everything and they get extra experience, it's inevitable these children are more visible in the school as a whole. The school needs to be careful though to encourage the quieter ones to come forward, and I think say them all taking part in each assembly rather than just a few star performers with the rest singing at the back makes a really big difference.
I agree with cory too- mine often have very minor parts in plays (the last one was non-speaking!) or come 4th out of 4 in the swimming competition, but I always encourage them to put themselves forward and try hard, and to cope with disappointment with this, in fact, I think competitions are good for this because it's very important that you can accept losing, or getting a low score, or coming last (as my dd did in all her races on sports day) but keep going.