I would try to join the fringe of a group/club or ask my Mom to get me lessons in X outside school and try to speak to one or two people in that smaller/less formal environment. Or, I would take up instrument/join the chorus, a group where I would have to interact. Can you force yourself to ask someone you admire to be your partner the next assignment that requires a partner at school? I would try talking to the person next to you in homeroom or a class before it starts, just some comments about the homework or borrow their notes or something?

Go with your mother to the grocery store to help and talk/interact with the clerk; get in the habit of talking to safe strangers like that who you won't see again/any time soon and it will help your courage and you might find it easier to start to talk to others at school.