I recently was able to get marking pencil out of a quite quilt that had been there for over 20 years. I had a little bit of regular pencil, but mostly a red/pink chalk marking to deal with.
I washed several times (with woolite) and I used shout sprayed directly on the markings. It came out almost completely -- because I know where it was, I can still see faint marks, but seen as a whole it's fine.
Good luck!
RST