First, I am proud of you for doing the right thing and telling your mother. That is the first step to addressing your eating disorder and moving forward to a healthy life. Since your mother doesn't seem to want to acknowledge that you have a problem you have a few choices. You can talk to your school nurse or counselor. They should be able to give you information to get you help. They may need to contact your mother, but that is what SHOULD happen. If you would rather, ask your mom to make an appointment for you with your pediatrician. The doctor can ask questions about your eating habits and why you think you have a disorder. They can also refer you to a therapist and specialist for eating disorders. Don't give up asking for help. It is there. Please keep telling people and asking for acknowledgment until someone helps you. Once again, I am so very proud you took the first step. Keep it up!!!!
I think she might be skeptical because people who have many eating disorders do not talk about it, they have a fear of gaining weight, conceal that they are not eating, and do not notice that they are getting thinner. Also, some people binge eat but, it's usually in private and they don't want people to know. You'll have to explain to her what you're going through. Rather than saying that you have an eating disorder, list the symptoms you've been having and ask her to take you to a doctor.