@frustrationcentral
It tended to always be the same children at DC's school - confident children but there were others that would have had a go (not mine, they hated the idea!

). Children's parents always usually the ones who were friendly with the teacher, or the ones who volunteered reading/trips or governors.. could be a coincidence but used to be a major cause of moaning amongst parents who's children never got a shot
As a teacher, I don’t necessarily disagree. Parents who go the extra mile, coming in religiously to read with children, helping out on trips etc. can 100% be relied upon to help their children learn lines etc.
We also have parents who can’t do any of the above due to work commitments but every reply slip is given in the next day, homework is always given in, and it is evident they read with their child. They too are likely to have children who will get vocal parts as we know they will be supported.
Unless your child is a confident reader and self driven perceived parental ‘attitude’ to supporting the child/school will always be a big factor in who is chosen to learn lines.
Saying that our Mary and Joseph never spoke so I often gave those parts to children with good attendance, moderate confidence, but who were verbally shy or were under SALT.
My kiddies with SEND who needed support were cast as a king/sheep depending on their needs. Kings were the kiddies who could manage sandwiched in between two other children. My sheep were the children who needed more support from their 1:1 and we’d make little lamb fiddle toys for them. I have had a Joseph with SEND who had amazing sequencing skills and could remember where to move perfectly. He was amazing.
Everyone was given a part, but children who had poor attendance were given roles which wouldn’t be impacted by them not being there. They might be part of a flock of sheep or chorus of angels.
Children with ‘challenging’ behaviour were often located close to the TA supporting the SEND child so she could support their behaviour too. By default they were generally sheep/shepherds.