I can hear your frustration and I do understand that. I am not a teacher but I can’t think of a perfect way to sort it out so that, through all the years of a particular school, each child gets the opportunities they want AND the plays are reasonably well performed.
Do you want the school to have a published policy on how they allocate parts? Do you want a record to be kept for each child, logging how many lines they spoke or whether they danced etc. and for each teacher to check the record before allocating? How would this take into account each child’s preference? (plenty of people have said that they’ve discovered that their child didn’t want a big part) What about those children that say they want a part but don’t get support at home to learn their lines, or those that are off sick so much they can’t be relied upon to take the big parts, or those children who cannot speak clearly enough to be understood by the audience etc? What about those children whose parents complain, and then the next teacher feels under pressure to give them the best parts the next year??
This year (Y1) in my DD’s school they seemed to go for a ‘middle ground’ approach. The teacher discussed the roles with the children and they expressed interest, and clearly this influenced the parts that were given. That concept was good. In reality one of the narrators was not ideally suitable to the role: she was shy and panicked and said the wrong bit (and you could see her partner mouthing the words and itching to help out!), and one of the lead roles was played by a boy with something of a speech problem – it was pretty difficult to work out what he was saying, even though he did it confidently. Meanwhile a few individuals with small parts stood out for their confidence and clear speech. It would have been easy for the teacher to have given the big parts to these children, and no doubt the play would have been much better. Equally it would have been much worse if all of the shy children were forced into big parts. What is the right answer? I don’t know.
I think that ultimately it’s just not that important. I’d much prefer the teachers to be spending time creating inspiring lessons than worrying about who should get which part in a play. If my child were really keen in drama and not getting parts I’d get them into a drama group.