iBCC equivalency for US grades

I had a question about the deduction of 15-20% from American high school grades. In the US, there are no numerical marks, there are only letter grades (A, B, C..) and a Grade Point Average (GPA) for each year of high school. For example, if you have straight As (above 93%) in all your classes the entire school year, you would have a 4.0 GPA. Would IBCC somehow deduct 15-20% from GPA and letter grades?