If she's not mean or otherwise earning people's antagonism, I have some questions, because Asperger's Syndrome seems possible. It's much easier to overlook in girls.

Does she have a history of being bullied or ostracized? Does she have a history of extreme special interests, and does she have a tendency to talk about her interests without noticing whether anybody else cares? Does she lack interest in what the crowd is into, like their fashions, TV shows, and such like, and identify with other things that others don't care about?
Does she have an unusual sense of justice, and is she a poor liar? Does she tend to be rigid in her thinking, compulsive about organizing some things, and yet unable to get organized?
Is she basically anxious all the time?
Does she have certain physical signs such as:
sensitivity to light, especially bright sunshine?
a history of belly rumpus with no good reason?
sensitive or insensitive to heat or cold?
inconsistency in hearing - for instance sometimes not noticing when someone talks to her, but picking up a conversation in the next room?
unexpected reactions to drugs, such as pain medicine being ineffective or coffee being calming?
Is she picky about food and like certain foods to an unusual degree?

If a fair number of these things are true, then Asperger's Syndrome should definitely be looked into. It's not such a big problem, as long as you and she are aware of it, so that you and she know to include it in your thinking and be sure that the school doesn't ignore it either. It's a real problem if that's how it is and you act like it's not, the way it is with any situation. It's not a big deal that I can't play basketball, but if I didn't know that and felt like I had to, life would be bitter indeed.

It is not uncommon what she is going through...and it may help her to know that. What she also might need to understand is that if she needs things to change she needs to make some changes herself. I have been through this with my daughter...and it has been hard on her..but as shy as she was she found the courage inside to make small changes to make friends. It could simply start from having lunch outside of the office and being brave enough to go up to someone else who also is alone and sitting with them. It takes a lot of courage ...i feel for your child...I recommend very baby steps in her changes.

Our daughter faced similar problems. She had been homeschooled for a period of time due to an illness and even kids she knew from the past now presented a seemingly impenetrable wall of already established friendships. We live out in the country and the only families within 2 miles have kids who are not less than 2 years apart from her in age. Our daughter loves to draw, likes anime and likes singing. We tried a church choir, a dismal failure as far as friendships and sadly ditto at church youth group where we were relatively new when our rural church was closed by the diocese. We tried an art class and it was too structured to allow interaction between students. Where we ended up having wonderful success was at our local library with their teen programs. She now is a happy member of both an anime club and a monthly game night club with 2 friendships close enough to involve exchange of birthday gifts, social visits to shop at the mall or go to movies etc. Your daughter sounds like she has a sweet spirit and is willing to help others since she pitches in to help at the school office. Our daughter also found some comfort in finding online community interested in art where she found far-flung friends who supported her interests. This has really helped her. Where are her old friends from the smaller school? Can u reconnect online to give her some ongoing support from them? Kids DO need to talk to other kids but that so often takes place online or by text that maybe just reaching out to someone she already knows would relieve some of her anxiety while she makes this transition? Best wishes.

Although you cannot make her join any clubs continue to encourage her to join one that she shares an interest in. Remind her that they all have that interest in common. Also let her know that if she goes and doesnt like it she doesnt have to return. Chances are if she takes that first step by going into the club she may actually make friends.

I strongly encourage you to read - Reviving Ophelia: Saving the Selves of Adolescent Girls www.amazon.com/Reviving-Ophelia-Adolescent-Ballantine-Readers/dp/0345392825
"From Publishers Weekly
From her work as a psychotherapist for adolescent females, Pipher here posits and persuasively argues her thesis that today's teenaged girls are coming of age in "a girl-poisoning culture." Backed by anecdotal evidence and research findings, she suggests that, despite the advances of feminism, young women continue to be victims of abuse, self-mutilation (e.g., anorexia), consumerism and media pressure to conform to others' ideals. With sympathy and focus she cites case histories to illustrate the struggles required of adolescent girls to maintain a sense of themselves among the mixed messages they receive from society, their schools and, often, their families. Pipher offers concrete suggestions for ways by which girls can build and maintain a strong sense of self, e.g., keeping a diary, observing their social context as an anthropologist might, distinguishing between thoughts and feelings. Pipher is an eloquent advocate. Psychotherapy Book Club selection;"

I agree with SoCalGal - seek intervention immediately.

What were her friendships like in the smaller school? DId you recently move so that she doesn't know anyone or are other kids from the small school there?

Friendships change as kids move from junior high to high school but a student who sits in the office instead of with other students is a student who may need some intervention.

No my cousin works at the school she help's her out answer phone things like that. I'm just goingto pray on it and leave it along

sounds like you have a very normal daughter. my daughter is 13 and experiencing the same growing pains. It appears she has a great support in you. Many kids don't have that type of emotional support. I am looking for social activites as well for my daughter. I hope she keeps in mind, this is only a season of life and it will pass, and know that there are great things ahead. I believe God allows us to experience events in our lives in order to be a bekin of hope for others. Remember attitude is everything, it is a choice.

tell her when she goes to her classes find a class mate that sits beside her and try to start a chat before the teacher start to speak. that help me when i was in school.

Where does your daughter go to school? Maybe instead of trying to get lots of casual friends, she can concentrate on finding just a few close ones that will be there for her. My son doesn't talk alot, but he found a small group of kids that he fit very well. Given time I'm sure your daughter will find the perfect people to be her friends.

When you say your daughter stays "in the office" during lunch, whose office? A teacher's? What does she do there? Are there any other kids in that office at the same time, that she might pair up with, and then they walk into the actual cafeteria to sit and eat together?
Joining after school clubs and activities really is the best way to meet kids who share similar interests, and become friends. What kinds of things did she like to do in her old school, or what are some of her favorite subjects? (For example, if she likes English, and writing, she should consider joining the school newspaper or yearbook. If she likes drawing, she should consider art club, etc.) If you're really worried about her, call her guidance counselor, and see if that person can give her encouragement; maybe the counselor can introduce her to the teacher in charge of whatever club she might like to join, in order to "break the ice."

I also have a 15 year old daughter and she also came from a small school and by the way their are no close family members.So making friends came with time.M y advice to my daughter was to start talking to people at school that maybe you have classes with and join clubs.Be approachable and you may have to be the one to start the conversation.