That is exactly what DS's kindergarten was like last year, too. We were very very frustrated, particularly since we had just moved to the 2nd highest-rated school district in the tri-county area specifically for the schools.
Our kindergarten teacher gave him a differentiation plan that included participation in the lexile reading program, his own books, and eventually his own math/spelling/language arts. However, the differentiation was way too chaotic in a kindergarten classroom. We also considered the school's recommendation to move him up to a 1/2 combo, but the biggest problem was that he was still far ahead of 2nd grade in some areas yet still had the gross motor skills of a 5 year old boy and the fine motor skills of an advanced-5 (but certainly not 7!).
We quickly discovered that the K schoolwork was a joke (worse than his old Catholic preschool), the "average kids" were surprisingly far behind (can't recognize an "I"?? Can't count to 20?!?!?), there were tons of immaturity/behavioral problems by other Ks that always diverted the teacher's attentions, and that he could do so many more productive things with his time outside of that classroom.
We pulled him in January to homeschool him (through a charter, with tons of workshops and co-ops), and we never looked back.
We were very lucky that DS was SO far ahead of the curve that they had to differentiate. Unfortunately, with 24 in a class at that age, the teacher's time is stretched so thin that we found there isn't a whole lot they can do. Our teacher and principal were extremely accommodating to our wishes (said they've never had one like him before, and had the superintendent involved for us, too), but at the end of the day there just wasn't anything that worked well.
I truly hope you find some answers. It is so heartbreaking when the solution isn't right ahead of you, but keep persevering, talking with the teachers, and finding enrichment programs. and hopefully it'll all come together soon.