I need help standing up to my friend?

Im in middle school and my friends all have a group and there is this one girl named alex that is in it she is not very nice to me. The rest of the girls are awesome and are very nice to me, but she is kinda the leader and im afraid to stand up to her, she always calls me dumb and stupid cause im in special ed (Actually her best friend is in it to!) Jessica is Alex's best friend and she is in special ed as well, but she treats her like the princess and I feel like a tag along around alex. Im extremely afraid to stand up to her. Here's another great example our cl is going on a field trip to catalina island me and Alex originally where supposed to be partners the next day I found out she ditched me!!!! without even explaning to me why!! please I really need help I need to learn how to stand up to her!