Ok, so I am both a parent and a teacher. I genuinely think, when it comes to picking parts in the school play, you go for the student who is going to be best in that role - for a large part you need to know that they will be loud, clear and be able to handle the pressure (it's really scary to perform Infront of a big audience so you do need to pick those children who you know will be up to the job and not being too scared to go on. Otherwise it's a disaster) They also need to be able to memorise lines well. I would never pick the same child over and over just because their parent helped out on the PTA. We are also always rooting for those children who may struggle academically or find things difficult if we think they'd get on really well in a play.
My eldest daughter, since reception, has often got lead parts in plays or been put forward for things that involve public speaking. I think she probably was one of those kids that parents perceived was always being picked. She did seem to be popular with the teachers, but in fairness she was always very polite, easy going and well behaved.
She's quiet but confident and has a loud clear voice with really good expression. She's really reliable and puts herself forward for everything and tries really hard. However, when it came to the sports teams she was not bad, but not the best, so would sometimes scrape on the team, sometimes not. As much as I would feel disappointed for her, I did have to accept that they're going to pick the best players if they want to win a match against another team. So, when all the sports was going on we went through a spare where she was hardly picked for anything so I can totally empathise with how it feels when your child never seems to be picked for anything.
Interestingly, she's moved into a new school for Year 6 so I guess the teachers don't really know her very well. They've been auditioning for a musical and she's had her heart set on the lead role - she's quite good at acting but I would say there would certainly be better singers than her. She's absolutely put her all into the auditions and has amazingly, got the lead part. I don't help out on the PTA and the teachers don't know us at all (unlike her old school.) Interestingly though, my younger daughter, who has a beautiful singing voice, didn't even get through the first round of auditions for the talent show. As much as I wanted to say it was favouritism, she can be a bit temperamental and sometimes gets overcome with shyness, so I think this is what happened.
In honesty, I think it's always going to be annoying when one kid always gets picked for things over others. But sadly there usually is a reason.