I liked @Willow Rex's answer quite a bit, and I think it can solve your problem. However, there's another aspect to your question that I think can be addressed if you so desire. Since you already told the teacher in charge and she dismissed your concerns out of hand, then discuss it with the principal. If you get no results there (they really should take this seriously), I think you can speak with the students directly.
Your body belongs to you and no one should touch it without implicit or explicit consent.** If you're averse to hugs, be it for whatever reason, it's a violation of your physical and psychological boundaries when people hug you uninvited. Ten year old girls (unless they are special needs) should have no difficulty understanding this.
They were probably told that strangers should not touch them, and no one should touch them inappropriately, so they do understand something about the invasiveness of touching. It's time to expand their horizons (kindly and lovingly) about your discomfort with hugs. They should be aware that their intrusion into personal spaces don't always feel like gifts to the receiver.
The next time one of the girls hugs you, ask if you can speak with her alone for a minute (alone meaning in public but where no one else can hear your conversation.) Tell them that while you like them, really enjoy teaching them, and appreciate their affection, you don't feel that hugs are appropriate, and they make you uncomfortable. Again reiterate that you appreciate that they like you, and you like them as well, but a simple "thank you" will do if they want to express their appreciation for your attention and your teaching. They are becoming young adults now, and should respect other people's personal space. Finish by telling them you don't want to hurt their feelings, that you enjoy teaching them (and whatever else you can honestly say), but it's no hugs from now on.
They will probably feel embarassed or rebuffed, so pour on the smiles and kind words. Greet them with a smile whenever you see them or when they take leave your class. That should help.
**I love animals, especially dogs. When I see a dog I'd like to pet, I always ask the owner, "May I approach your dog?" Dogs have a personal space, and violating it might mean a nasty bite, or the dog might be a service dog, might be sick, or maybe the dog just doesn't like to be touched by strangers. While most owners say yes, you might be surprised by how many say no. If people should respect a dog's space, it should be obvious that they should respect a person's personal space even more.