http://admission.stanford.edu/application/freshman/transcripts.html
“We will focus our evaluation on your coursework and performance in 10th, 11th and 12th grades, primarily in the core academic subjects of English, mathematics, science, foreign language and history/social studies. We want to see that you have challenged yourself by taking accelerated, honors, Advanced Placement or International Baccalaureate courses if they are offered at your school.”
I don’t believe the same is true of the rest of HYPSM. That being said, I got into H with some freshman year C’s, although that was some years back now.