I am glad that you understand, Stacie, what I was trying to explain. The reason that heterogeneous grouping doesn't work a lot is because it is not done the right way. Why? Because it is hard and it takes a lot of work, monitoring, re-teaching, assessing and having tiered levels of assignments and assessments for every concept you teach. I don't want to bad mouth an entire group of people, but the school I last taught at was a lot like you are describing. Fortunately, the administration, over a 10 year period was able to get rid of a lot of the offending teachers. When I was hired, I had 22 years of experience and the other teachers saw me as the devil... I just kept doing what I had been doing and eventually, after about 5 years, I was surrounded by colleagues that were doing things the way i did. High expectations for all does work, but very few schools are willing to make the effort to do this.
It is great that you finally got them to listen. However, you very well may have to work full time to afford private school if things do not work out. I worked the whole time my kids were growing up and they certainly did not suffer.
As for the visualization, yes, it is the first strategy to use for comprehension. But, make sure your son is exposed to all of the strategies as i think Veronica listed. They are taught in order of the amount of inferential reasoning needed.