I am sorry, blackcat, but it looks like you need to come up with creative solutions if you plan to stay with this school.
If the teacher is not doing any meaningful differentiation, then there is not much you can do about it without straining the relationship.
If you are an active volunteer, you can ask the teacher if you could take the gifted children (you could call them "advanced" if you like) aside and work with them on challenging content - like difficult spelling, advanced math, reading groups etc.
Last year, my son's teacher always claimed to differentiate, but nothing seemed to happen. She said that she could handle the differentiation when I suggested that I bring it up with the administration, but then, she was always looking tired, worn out, disorganized, hassled and trying to keep on top of things because she had a mixed class of K/1st graders and the effort to differentiate seemed too much.
I put my foot down firmly once and she conceded that she had no time (I read that as unwilling?) to put in the extra effort to create special curriculum/worksheets. So, she allowed me to make my own work packets at my son's level and send it in his backpack every day (it is another story that this strategy failed). See if this method works for you.

Another easy differentiation was to allow my son to attend "library reading time" with the librarian with a higher grade and permission to check out higher grade level books.

And they differentiated a lot in the Tech Lab time too (I was an active volunteer there and took my son aside in the lab and taught him independently).

As for the unchallenging homework - our policy has beeen to do it irrespective of how easy or "silly" it was. The idea was to teach my child to do all his tasks and not to pick and choose what he can do - we all do pointless tasks daily whether we like it or not. What we found was that this kind of homework, when done before afterschool activities, gets done very quickly -
Me: "Son, the sooner you finish your math homework, the sooner you can head out to Karate class."
DS: "Sure thing!" (where otherwise it would take a whole lot of whining. You can insert your son's favorite activity here)

As for the SM afterschooling, I gave up on asking DS to work on it after a long day in school (he has problems with writing too) and it takes forever to get even one page done in a day. So, my creative solution to this is: he works through the problems verbally and I scribe for him. This way, we get 4-5 pages done in one sitting while I can correct him as he goes and we review concepts while doing work. We have gone through one whole level of SM books (including IP and CWP) this way.

As for telling my son - I told him that the teacher had rules that she had to follow a certain curriculum and also that in a group setting, we have to all wait for others to catch up. But, we could always advance while afterschooling.
Good luck.