I have tried this and it works! Make a solution of 3 fluid oz. rubbing alcohol, 1 fl. oz of water and 3 or 4 drops of liquid dishwashing detergent.
Place in a squeeze bottle and shake well. Wet the corner of a rough washcloth with the solution and rub over the pencil markings. Wet a second cloth with clear water and wring out; rub over the treated surface to rinse. Let the quilt air dry. Lines will disappear entirely when the quilt is dry. Heavy pencil marks may, however, require a second treatment for complete removal.
Hope this works for you.