Just give it to her straight, try not to laugh if she does (if it's that unbelievable). You should have a relaxed tone not angry not sad not excited, just an honest, relaxed tone. If she refuses to believe and starts saying no you're not, keep saying that you are, but don't get annoyed. If she wont believe it's her problem. Overall just be your honest self when telling her, she doesn't know your life, she can't decide whether you are bulimic or not. :)
I really want to tell my teacher that I am a bulimic but I'm scared that she will think I'm making it up because I'm not too fat or too thin, and its hard to believe what should I do?
One thing that you should be able to seek solace in is the fact that teachers are there to support you, nourish you and be there for you. It is what they have spent years training in.
A person who is unable to feel some level of empathy is unlikely to want to pursue a career in the field, so find comfort in the fact that you have peers who are going to take care of you during this time.
It is so vitally important that you communicate with your teacher, or another adult that you trust, exactly what you are going through. Bulimia is a very dangerous illness and is a psychological problem first and foremost. It requires that you undergo some CBT (cognitive behavioural therapy) to realign your thought processes in order to overcome the illness. In order to get to this stage, it is vital that you confide in an adult who can help you.
Your teacher will never judge the severity of your circumstance on your body weight. The truth is that a persons physical condition is not always indicative of their inner turmoil, and as a grown adult she is surely aware of this. So don't fret, and try to speak to your teacher as soon as you can to get on the road to recovery as fast as you can.
Try the NHS website for Bulimia to see if it can help you further.