I'm in church band. Band leader gives a set of songs to practice. After rehearsal, they decide which four songs out of the 11 they want to play on Sunday and the key they want to play in. This is really annoying sometimes as some songs are really weird to transpose, and those songs tend to take a bit to get down versus your generic 1 6 4 5 songs. Worst of all sheet music isn't always available.
As said by others this is unacceptable and unprofessional. This is not a paid position and a perfect example of "your not being paid so they don't give a rats backside and is a complete lack of respect".
I have been involved in church music since I was 9 years old and never once been treated such with disregard and lack of respect.
I have been the bass player since 1997 in my current church. We get at least a 13 week schedule of music. The leader knows his range and the range of others. We are provided with SHEET music, SATB and chords, in advance of rehearsal if it is new music to us or you are new to the group, and audio samples. We are provided with a sheet showing the list of music for the Following Sunday on the prior Sunday with the "final list" so that we can practice and be ready for rehearsal. We often have suggestions and there are also corrections hence the final list. We put it together at rehearsal, we do not learn our parts at rehearsal. Learning your part at rehearsal is lack of respect for the other volunteers time.
I currently use MobileSheets to organize and display my Music. (One of the best apps I have, what a joy). I currently have 520 different pieces for the Church, 8,000 pieces total. I only scanned perhaps 60/70 % or the paper versions I had for Church as we were unlikely to do the others again.
The first year was real rough and the second only a little better. Every week it was 8 new pieces (to me) of music. No disrespect to studio musicians, but this felt like studio work to me. 3 days to learn all 7, rehearse, and 2 days to lock it down, and run through the changes on Sunday as reinforcement before leaving the house. Then you are expected to to stand up and play one song, sit, then stand up again play another, sit, repeat and rinse. It was tough getting into a grow and warmed up. But I learned to do it. I enjoyed the challenge, I was respected, and I was making a difference. As I entered the 3rd year it got easier, I actually noticed music started to repeat, and I was thankful I left plenty of notes and scribbles on my music to guide me.
If we are doing music new to the group, we always rehearse it multiple times and if the key is wrong we distributive the music in the right key before continuing. No surprises when it's rehearsed or use on the following Sunday. Sometimes Key changes happen with new stuff especially if more contemporary or modern.
The people in our group have been together for ages literally. People do come and go as life changes. But we make sure they know they are welcome, and we help them get up and running. We will spend more time going over things we already know to help them get up to speed, especially with harmonies.
One thought, if you don't already understand basic music theory (it's much easier than it sounds), then learning music theory will help a lot. Also learn the music for Sun after rehearsal, that way you only learn what you have to. Run through all the Chord changes on all the music before rehearsal so that you have a rudimentary handle. Now if rehearsal is Saturday that does not help much.
But you are going to have to talk to the WL about this, I learned in grammar school that if I don't understand something much of the class doesn't either, so I always asked. The rest later admitted they were too chicken to ask. So bring it up, make it a group conversation. If the others already know the music, then ask why don't they have more of a clue, and why chase people away.
It might be a good short term experience too. You might learn to make this happen and that is a valuable skill. But this could also be a red flag for the WL. This is what you see now. Keep your eyes open, and only stay if it is a good fit, and it's not causing you too much stress. New process is always a bit stressful.
If you can get the WL to act professionally and reasonably, then it's a great experience.