DC's school does 4 plays per year, 3 involving 2 year groups, 1 involving just Year 6.
For all of them, children get to put down their choices - to have a big speaking part, a small speaking part, to do a dance or to play a musical instrument. Everybody sings, the classes work in big groups to make props / scenery. Every child except those playing musical instruments (so perhaps 100 - 110 of the 120 kids involved) appears on the stage in a group or on their own to 'do their bit' whether 'their bit' is part of the narrative, a dance or a verse of a song.
'Speakers' of both kinds audition, the best get given the parts.
DS's first option is always a small speaking part, which he generally gets, so he tends to have a couple of lines.
DD changes her options every year - she's been a dancer 3 times, a small speaking part once and a HUGE speaking part once.
I teach elsewhere, and can't get to PTA stuff so it's definitely not because I'm 'involved'.