I want to point out another negative side to this, I was used to getting a good part in the school play, I then went to high school and my confidence took a battering because suddenly there were 100s of kids trying out for each role, older years were favoured and the drama and dance teachers had their favourites. Most children went away with nothing
I think my primary school did a lot right though, I'm not sure about normal assembly, I think we stood in a line and said one line each. Our school play was really good though, including props, smoke machines etc. The younger classes would stand up together and do a song or something, dressed as flowers or something so they were scenery too. The older years would do this too but you could also audition for a part, everyone who auditioned would be given some sort of speaking role.
The cast would audition at break time, the occasional unimportant lesson etc, the rest of the school would have a few lessons practising their bit and entering / exiting but that was It. There was no sitting around, even the main cast would only be there if you were in the bit they were rehearsing. We did maximum of 3 full rehearsals
The choir always sung too, and that was open all year to everyone, you could even have a speaking role and join up for the choir bits if you were normally a member
I think including all years meant you grew up a part of it so it wasn't as scary auditioning later, and as all of Y1 might be daffodils (for example) singing together, everyone was equal