I'm to dependent on others?

I'm always with my friends and when I'm forced to be by myself at somewhere like school without them I feel powerless and also people pick on me when I'm by myself but when I'm with my friends it doesn't happen. Theirs just so many people in the school that try to get to me and push my ons. I just feel weak when I'm by myself. How can I change this? How can I change the constant fear of me having to face things alone?