It wasnt just the back of the head either, I had no beard at that point, they covered my face too. It was all washable marker, but after a trip to the store like that to get some food etc for dinner, I showered it off that night....the water was brown from all the marker....but...its something that everyone remembers to this day....and memories are worth more than any embarrassment (especially my embarrassment)
I had the kids do it at a going away party for the pre-kindergarten teacher...some kids were like, "are you sure this is ok?" I was all "go for it, have fun" Had like 4 or 5 kids drawing on my head/face at a time at some points...