Before the first rehearsal, your choir sings every chord in tune. They never disagree or have a bad rehearsal. They don’t check their phones in class.
They always remember pencils.
In short, before the first rehearsal, your choir is perfect.
But wouldn’t you give up a fair percentage of your dream choir’s perfection to get a little time making actual music with your real, imperfect, wonderful choirs? So don’t be discouraged when reality strikes and your choir is less than perfect.
Perfection only happens when the choir isn’t singing at all.