I think the teachers' are damned if they do and damned if they don't - if they are in charge of a class assembly, play etc... I guess they (and the children) want it to be successful and if they know they have reliable children who will deliver it makes sense to choose those ones. It is a bit annoying but maybe after viewing the children throughout their primary education it becomes apparent that those children are chosen for a reason - they speak loudly, clearly and are sensible. This happens in DS's class - DS mumbles and speaks quickly so doesnt get the 'big' parts - used to annoy us but now I realise that public speaking is not his thing. Also, often those who are the confident ones tend to put themselves forward and thus it becomes a self-fulfilling thing...they are confident, they put themselves forward for the lead parts, get more confident, get more opportunities etc....
Whilst it is endearing with the little ones when they get all shy and forget their words in public, as they get older it becomes embarrassing for all concerned but especially the pupils, who maybe don't have the natural confidence or the inclination to do public speaking related things, when they forget/stumble on their words etc...
Interestingly, there seems to be a link with another thread about the youngest borns in classes being regarded as low achievers. Thinking about both DCs classes (and I am making generalisations so don't shoot me down!) it does tend to be the Autumn born ones who are the confident ones and do get chosen for main parts - read into that what you will.
Also, you are right, Spiderpig8, about the parents' factor too! Middle class parents can be very pushy in school and out and they will bring their children up to put themselves forward - and so it goes on.... Again, in DCs classes it does tend to also be the more middle-class children who get chosen..... - is it confidence or that natural-entitlement thing which rubs off on the teachers with whom these children come into contact.
I guess there is also another issue about the bright and the popular children getting extra confidence from being bright and popular thus making them and the teachers have greater belief (not always correctly) in their skill-sets....