Karen, while marker looks like it is really tough to get out, it is really easy if you use the right solvent which is alcohol. Go to the drugstore and buy a pint of denatured alcohol. This is relatively cheap and for whatever reason, cheaper than the same alcohol sold as shellac thinner. You can even use cheap perfume in a pinch.
If your desk has a shellac finish, you might be in trouble but usually they are polyurethane or varnish, which are not affected by alcohol. If it is unfinished you will have to let the alcohol soak in to dilute the stain. Then just wipe it up and all the color should come with it.