Rather than simply look at the grammar, I would like to suggest an overall revision (keeping to your words as much as possible). Here is how I would say it: 6 May 2015 XYZ Subject: Pay Scale Correction Dear sir: Although I have been promoted from Middle School to High School (where I teach mathematics for grades 9 and 10), the basic pay I received for the month of April continues to be the school's rate for middle school teachers. I have always worked to advance the best interests of the students and the institution since I joined the school in March 2012.
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