I'm happy to say that at my school, we judge children based on what they are taught in school and not on what they may or may not learn outside school time.
For example, we teach academic and sporting skills to all, which makes it fair to choose a team based on ability for these areas. Furthermore, we field as many teams as there are keen children, no child is denied an opportunity to play if they would like to.
Drama is not something we teach as a skill in the same way, rather more as a fun way to explore ideas. Although we do have a number of children who take Speech&Drama outside school, we don't promote these children over those who, for whatever reason, do not take such classes when it comes to casting a play. Everyone gets a part in a production who wants one, (our children appreciate on stage as well as off stage roles) and above all, we keep it relaxed. How could we judge children based on something that happens both outside school and outside their control? It's not representative of my school's ethos and would be a sad end to what should be an unqualified happy memory of year six.
In short, the production is about the experience of taking part and enjoying the process, rather than simply showcasing talent which we can't take any credit for.
I understand completely why you are concerned about your daughter's role in her final production, suggest you chat with the Head to see how she can be more fully included as a leaver.
Barbara.