The high school is interested in letter grades because your son's high school transcript will have a letter grade for each semester of the classes that he completed. The transcript will be sent to the colleges he applies to. Those letter grades are what is used to compute the overall GPA which is also on his transcript. Colleges expect to see letter grades. UW schools generally use unweighted GPA. Some colleges will use weighted, some will have their own procedure for recalculating GPA, taking out Phy Ed, Health, maybe even art and music classes for an academic GPA. But all of them will look at the transcript and see what kind of classes they took, easy ones, lots of honors and APs etc. Often when your son's guidance counselor writes a letter or fills out a form for college admission his senior year they will make a statement or check a box indicating the academic rigor of the student's high school courses. Most rigorous, rigorous etc. which is in comparison to what other students at the high school generally take. Those letters or forms are sent to colleges as part of the admission process. That helps level the playing field for kids who attend small rural schools which may not offer a lot of honors or AP classes.