What to do if my friends push me away?

My friends are always pushing me out of things like eg we are going to a safari park and there is like a group of 5 people I'n my class including me who are best of friends but know I think they are pushing me away and obviously 5 is a difficult number to split your selfs into 2 pairs and so 2 of my friend have paired up wit eachother and the othe 2 have aswell and ime just left by my self and I came home really upset but I really don't know what to do because it's really hit me because that is not te only thing they have done to me I also found out 3 days ago that they have been backbiting behind my back and have been saying really bad and upsetting things and ime really hurt I did come home upset but I did feel like crying but I knew that they wernt worth it and I dont know weather to approach them and tell tem that it's not fair that they have been doing this to me but the main problem is that they are big an the most big people I'n the class and ime like small and everyone wishes to be part of them ...please help .. :(