Why does my unofficial "girlfriend" ignore me?

ok, i met this girl about 8 months ago and weve become really close. we are in high school and are two years apart (that plays an important role later in the question) and i think she could be the greatest thing that has ever happened to me. i like her more than i thought i was able to at the beginning of our relationship, and as far as i know, she feels the same. i am the first guy she has actually like liked and pursued anything with. but i have noticed that she sometimes doesnt show this very well. when we are alone together, we are as happy as can be and we enjoy eachothers company very much. however when we are hangin out with our group of friends, she completely ignores me, except fo maybe a few smiles or glances here and there. i know it is not a matter of it "being me" because we are completely comfortable aroud our friends because we have the same friends. we are one big happy group, so its not like i embarras her or anything. it seems like she would rather talk to others and not talk to me at all. she even laughs with them like shes never laughed around me. its like she enjoys others company more than mine and i am 2 priority. i go out of my way to be with her and talk to her (it shouldnt have to be like that) but she doesnt respond to me like she does her friends. i cant make her laugh or smile the way others do and thats very frustrating. now the age part. she is 15 so the only thing i can think of is that she is young and still at that stage where she cares mor about her friends than she does me. the thing is, i like her SO much that i am willing to go through that. i am constantly going out of my way to reach out to her and she constantly ignores me. plus she never puts in the effort to do the same to me. i am always going to her. the most frustrating part is that she always says later, "sorry we didnt get to talk much". ithis makes me so mad that i want to just ignore her as much as she does me, and say that its always the same story. so my question is, why does she ignore me when we are hanging out with our friends. is it something to do with me or is she just not realizing what she is doing. and please, no crude answers.