Recently had this issue myself after years of never having a problem with pencil marks. After repeated washings (and NO drying) with all manner of stain removers with no success, I finally attacked the marks with dish detergent and a toothbrush. Had I worked on it longer I believe all the marks would have come out. As it was, after a few minutes the marks were barely visible and it had to be okay as I was out of time. It was being gifted the following AM. I knew the marks were there but no one else could see them. Good luck.