I am wondering if you have made an appointment to talk with your Principal...with your child...with the present teacher. When we are presented with changes...we often don't understand why or how it will really function in the new situation. Children will adjust and will gain so much from new situations when they have a positive attitude about them which can only come from their family. Sometimes combination grades are necessary...just as you said...as an alternative to something which is worse. School is not an individual place. It is a community with the needs of everyone in mind. If you want information regarding your options...the place to go is to your school administration. They are the only ones who have the answers.

What state do you live in?
There probably will be two teachers?
How many students will there be?
My daughter had a 5th/6th SPED class that worked great! Teacher had an aid - there were about 20-22 kids in total. The teacher made it work and worked well! This was in a public school. Actually my daughter was forced to focus more because when they were working on 5th grade the teacher focused on 5th and the aid would assist the students in 6th work on their project.
check for rights based on state starting wtih:
www.ed.gov/index.jhtml