Short answer is they don’t. You’re starting off on a blank slate in that any credits received prior to PEA do not reduce the PEA credits you need to earn at Exeter based on your entering grade. Your past grades do not figure into your GPA. Your math and foreign language options are based on your placement tests, not past courses. You take the standard 10th grade English course, and your choice of 10th grade history. Your science option is the only one that will take until account your past science course.
But really, don’t worry until you are actually accepted.