The same thing happened to me.
I asked for advice on this Quilt Board as you are doing. Below is my post and the answers I was given. Just click on the link and it will take you to my post and the advice I was given. I tried the Dawn and Oxi-clean. Also white vinegar and placing quilt in the sunshine. Did every mark come out 100%...not for me. A few marks remained, but they were faded so much that I was happy with the results. Good luck and please let us know if you have success. I included a picture of my quilt. Ps. The pencil marks on my quilt are hardly seen once the fabric dries.
Advice please for how to remove mechanical pencil stains from a quilt.