Would you be this frightened if she was friends with a boy? Kids at this age are trying out things to learn who they are. Because she's special friends with a girl does not mean that she's lesbian. It means she's trying out the concept. She may be lesbian but the guidelines for relationships are the same either way.
She also may be labeled a lesbian by classmates without her really understanding the significance of that label. She's different and subject to judgement. Kids can be cruel when they see someone different.
I have a 12 yo granddaughter. She has close relationships with her girl friends. They hug each other often, hang out together making close physical contact. Walk down the street with arms around each other. They are not sexual but I could understand that someone who wanted to hurt them or someone ignorant of relationships could call them lesbian. The physical and emotional closeness is a part of learning how to love and be close to someone. It's a developmental stage.
She needs your support in learning how to handle a close relationship. This girl may be a good person with which to learn. I would focus on helping her to have a good relationship. I would not focus on the lesbian issue other than to tell her that if she wants she can talk with you about what that means.
I suggest that you get her in counseling with a counselor who can help her with understanding herself and how she, with her limitations, can relate to people. Many people with Aspergers do have successful relationships including marriage.
Above all do not move her to another school. This is an issue that both of you will have to face. Going to a different school will not make the issue go away and it will make life much more difficult for your daughter. She has friends at this school. She knows the teacher and the school. It would be cruel to make her adjust to new surroundings. You know she has difficulty now. Multiply that by many times in a new school.
I suggest that you have some counseling too to learn how to deal with this with less fear. Educate yourself about Aspergers and people's abilities to get along in their world. Educate yourself about teens, sex, and love relationships. This is nothing to fear. It is a challenge that you can handle with love and compassion.
After your SWH. I strongly believe that you're seriously over reacting. This will not be the first time she is in this type of situation. You're only changing the local and adding much stress for her. I urge you to discuss this with the counselor. Also to get a second and a third opinion.
Removing her from one school will remove her from this one girl. However, their are many people like this in the world. She will gravitate to another one unless she develops coping skills.
I moved to the coast with my 13 yo daughter in part because she was making friends with kids who were involved in negative behavior, including drugs. She made the same sort of friends at her new school in a smaller community. Because it was a smaller community she had less choices in friends and activities.
You did most of the talking when you talked with her. You haven't learned much if anything about what she is feeling and thinking. She has to learn how to deal with all sorts of people and experiences now while she's young. If you make all the decisions for her now, when she's on her own as an adult she will flounder because she hasn't been allowed to learn as a child.