I have had a lot of luck in the past by observing the teachers of the next grade in the spring to figure out who would be the best fit. I then write the letter to describe what my child needs tailored to that person within the confines of the rules.
Flexibility is a good substitute for term differentiation
Compassionate is a good substitute word for "nice"
Classroom management is a good substitute for discipline
It also works well to have a verbal conversation with the present teacher about your child's needs. That person will have seen it first hand, even if it's not interpreted in the way the parent sees it. In our schools at least, they are key participants in the placement decisions.
