I would say if you are that concerned - as many are... then your choices are either move them to a smaller school. However, you hurt them in doing so for what you want for them. This seems to be about what you want for them - do they share your feelings? (some kids want more, some are happy and some struggle) I do not fault you wanting the best for them, but changing their schools changes their best friends and that is an emotional stress that lasts. Especially if they have been there since Kindergarten.
Another Choice: Homeschool selectively - you can pull them out if you do not like what you see. This involves tons of responsibility and you know your kids best - will they be able to do it?
Yet another Choice - Give them extra incentive study... As you are able to teach, then you decide what they are lacking, and make up for it if you are able to do so.
You will have consequences to your actions no matter what your decision will be here, however. Will your kids resent you for interfering on a routine they are in? Especially if they are not "all A's" or honors? If you remove them to another school that takes them from their friends, are you prepared for that backlash as friendship is very important at this stage for children? Especially a move in the middle of a year - it is very hard to start someplace new and have nothing to build on, and feel alone.
I have been there on that side as the child. I was moved around frequently so I understand the repercussions of new schools. I do not take them lightly... If you do not agree with the curriculum, then make up for it at home if your children can fit it in with homework (make it fun, of course) or let it go for now. (Please do not let this be just all about you... make sure they want it.)
I am sure that if you encourage your children enough, they will endure, be strong and come through life in ways that will surprise you.
UPDATE to your response...
Sometimes we have to know that we cannot always follow all of our friends, as you mentioned your oldest child has had a lot of friends go to a different middle school. What happens when you do this for him now? What message does this teach when he still does have some of his other friends there? I feel that it would teach him to follow all his friends in life - high school, college and work - and in the workplace, that is not really the best idea. (Been there, done that.). At least it sounds like he has some friends there still. You also did not say that he is not happy otherwise. Kids want to follow their closest friends, naturally... but if they moved across the state, would you?
You have not mentioned how their grades are, but even a small change (whether or not friends are there), can still trigger stress. What if they do not click with the teachers there for some reason? You are right...There are many things to consider.
As for the school, I feel this problem may not just be limited to Michigan. The economy is affected EVERYWHERE. What happens when / if you do move your children and the same thing occurs - the academics are not up to Your standards? Will you be uprooting again? I feel the school system may just have it's hands tied. I feel you are looking at it through your eyes only and not both sides.
I see you use the word "Compete".
I feel there is more to life for our children while they learn, other than competing. Too much competition with others is not the best thing as this is all about someone's ego; Not about the person or what is inside them.
My family also never had an issue with your district (based on where your area is.). We thrived and are doing very well, and I am sure as the years went on, the system did improve.