Annie you are being absolutely ridiculous.
Primary school children can't show talent in drama?? That's crazy. Do you think talent suddenly materialises aged 12? Someone with a gift for performing will stand out from other 5 year olds aged 5 just as they will stand out from other 15 year olds aged 15. I agree with you that some children will not come into their own and show talent until later but on but other children are clearly gifted from a young age.
My gifted and talented drama group currently consists of 3 Y3s (youngest I can go without ferrying children from another site but I know the current Y2s are a very strong year group from overseeing their production at Christmas), 2 Y4s, 6 Y5s, 8 Y6s, 4 Y7s and 3 Y8s. This group of 7-13 olds are fantastic and they get so much out of working with each other for one production a year than just with their own year groups. It's an exclusive group because they deserve that chance to develop their skills and experience just as much as an A team in Sports does. I won't be talking to the cast of my current Y7 production about the nuances of effective acting in the same way because, with that one, I'm just aiming to get everyone up on the stage, knowing their lines, facing front and having fun. Two completely different shows for different purposes. The parents will, I hope, enjoy both of them but my G&T show will be good from a non parents point of view as well.
The most upsetting comment I've ever had from a parent re casting was 'we [as in parents of the school] don't come to these shows to see your shows your know, we come to see our children. X is only in 3 scenes, I might as well just come for 15 minutes then walk out. I you can't included them properly we're not interested.' I made the right noises but, having put 4 weeks of my life into that production, I left the room and cried. It is hurtful when parents can't be supportive of their child, the other children and the school. The vast majority of teachers are doing their absolute best to give everyone a role to suit them. There may be some who show favouritism, I don't know every teacher in the country. But I've certainly never come across it.