Coach the girl through it, explain about teeth, how it hurts, and remind her how great full you are she has your dick in her mouth.

I had a girlfriend when I first entered college who was only willing to preform oral sex. The first time she did it, I explained that her teeth rubbing across my shaft was really starting to hurt, and explained how to cover them with her lips. She would do it for a few seconds, forget, and start using her teeth again. Every time I got a blow job from her, it would be the same thing. I would remind her, than she would instantly forget. Needless to say, those were the worst blow jobs of my life, I'm surprised she didn't leave scratches.