It isn't that peculiar a situation. If you want to keep him as your instructor then you need to take a couple steps.
You can't force anyone else's behavior but your own. The thing I do when this happens is to make sure that the woman understands you aren't a threat. Don't expect that he can do this for you.
Talk about other people you are interested in that aren't him that are within your dating range. If you have a boyfriend bring him in so she can see him. Compliment her and compliment his choice in her even if you don't particularly mean it. This is about insecurity. Make her feel secure and it will stop.
I am known for being outlandish and outrageous but my instructor's wife is very clear that no matter how short my skirt I am not angling for him. It wasn't always this way but I made sure she understood. Just make sure she understands and things will be okay.