I recently discovered that common alcohol-based hand sanitizer works very well to remove stubborn white board marker. Not sure if it just works on my particular brand of markers or if the solvent is more universal, but give it a try.
Or, at least, I have done this successfully before now - so my current working assumption is "people are waiting too long after which it doesn't work", but since I like my white boards I'm unwilling to run the necessary experiments to test this.
So, my advice would be: if one brand of dry erase marker doesn't remove your permanent marker mark, try a different brand. I think I used Expo.
1. You have just applied the permanent marker
2. You have a fairly wet dry-erase marker
3. You blot the perm with the dry-erase so as to flush the site with ink
4. Attempt to wipe/remove
5. Do this fast
I've obviously not tried this with "hundreds" of whiteboards, but the one we have on the fridge at home works as well as the several size and types we have at work.