what do i do about my friend?

[mh17]

i have a friend that i have all my classes with this year, i knew her as an aquaintance before but now i got to know her really well as i also see her out of school. she is very close to my long time close friends too, so now we're all like one group of close friends. but i dont understand, why but when its just me and her in a class without the other two friends of our group, in a couple of classes she trys hanging out more and partnering up with other friends instead of me, when we're pretty good friends. then in those classes theirs no one else for me to partner with except this one girl tht i or no one really likes because she doesnt really talk to anyone at all. then i feel really bad that my friend left me alone like that, and i feel very bored and totally out of place all class long and i hate that feeling. i would never ever do that to my friend so why does she do tht to me?
i feel so lonely/bored/sad in these classes i have no one to be with and i totally hate that feeling more than anything in the world but what can i do about it. theres really no one else for me to be with..