I would talk to her about her goals and dreams. Maybe put it in the context of "what would you like to major in in college and are there any volunteer or school activities that you would like to try that might help you decide?" She may be an introvert. She may be shy. She may be not very popular. None of it is entirely horrible. I had a heck of a time when I moved to a new school in 6th grade, and it sounds like that's when she moved, too. It is hard when you are jumping in mid-stream. Theatre was my saving G.. It was not a goal to be popular (they were obnoxious kids). It was a goal to have a few good friends. My sister is a shy introvert and has a few good, but very close friends.
I would also ask if she wanted to go out and do some girl time things. Or even "hey, I'm going to x town. I wondered if you wanted to come with me and maybe stop by that bookstore in the shopping center."
And don't forget to tell her, "I'm proud of you. You don't give a lot of attitude and you're a good kid." She needs to hear that from the adults in her life.
By the way, the other kids could help her, too, if they wanted. Like say, "Hey, Jen, want to join us for lunch?" or tell their friends, "Yeah, Jen is really quiet, but she's really good with computers."
Or maybe Jen eats lunch in the library BECAUSE it's quiet and she can regroup in there. I rarely ate lunch out or in the kitchen at work. I preferred to eat at my desk, shut the office door and check personal email. Ask HER how SHE feels. She might say, "I like it where it's quiet and I can read." An introvert can be entirely happy in their own space.
I was that kid who literally walked between classes while reading (great peripheral vision). I graduated college, got married, had kids, have friends....maybe she just hasn't found her niche yet and whatever that niche is, make sure you're viewing her on her own merits and not by what the other kids do/don't do.