According to MN part allocation goes
Teacher's dc
Governor's dc
Active PTA's dc
The pushy mother who always speaks to the teacher at the end of the day's dc
Thus concludes all the parts that are worth getting...
Everyone else-chorus
In real terms it depends on the school:
In younger years some do it by age (which is why dd1 was Mary (oldest girl) and ds (June birthday) was an exceedingly lively shepherd who, with all the other summer born boys, got so thoroughly into character that they weren't allowed to carry their toy sheep in the performance having used them as offensive weapons in the dress rehearsal)
Some do it by audition
Some do it by teacher's choice
Some do it by asking the children what they want to be and trying to work round it.
Issues with all of them at various times.
They all tend to end with similar children doing the top parts.
Oldest-obviously does
Audition-the best dc tend to stay the best dc
Teacher's choice-they tend to pick the ones they've seen do an okay job before so the same ones get picked again. Also in ds' year they had a Joseph, so he inevitably got the part-despite I don't think he really wanted it.
It's all very well to think that it doesn't matter that it's a school play and it doesn't matter what they're like. But having been to one where the narrators didn't say any word that was audible until they spoke the final words together ("thank you for coming to our play") and another where the soloist was definitely not chosen for their singing voice-and was clearly very aware of this and embarrassed, and another where the very shy child refused to come on stage and teacher ended up reading their lines with them sat beside her, which threw several of the other children who were meant to interact with her.
Actually even if it means that my child doesn't have a good part, I would rather watch children that are doing it well and enjoying doing it.
What I do feel very strongly is that every child should feel they have a part and a costume. For a while mine were at a very large juniors and it inevitable came down to around 10% of the year having several lines, and the remaining 90% sitting at the back and singing songs wearing school uniform. I would have had a choir of 20 angels singing a song, and 20 shepherds/20 sheep etc. And having 10 shepherds with one line each rather than 2 with 5 lines.
Although one year they tried a production where each child had one line each. My dc weren't in that, but the parents who saw it said it was absolutely dreadful and took way too long, mostly because they all spoke through one of two microphones which were passed around-and the children weren't seated in order.