Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of principal],
Please accept this letter as a written request that [name of child] be transferred from [name of teacher]'s class on [subject] to a different classroom.
We feel like [name of child] is just not getting the education [he/she] deserves in the current environment and that a change would be better for everyone involved.
We have worked with [name of teacher] to attempt to resolve all the issues that have come up, but have been unable to do to the satisfaction of everyone involved. We truly fell that this is the last available option, and we would like to request your prompt attention to this matter.