I am sorry to hear that you feel shy about letting people know about your diabetes, as it must make you feel like you are keeping a terrible secret from others.
Please try and remember that diabetes is not something to hide from others and be ashamed of in anyway. You are a unique person who happens to have diabetes, and you will be pleasantly surprised by people admiring you for coping so well, and getting on with life in a positive way. If you start to trust your classmates more they will respond and help you to overcome your shyness.
Perhaps you could ask your teacher to arrange a short discussion on diabetes and you will be the expert on the subject, this will overcome a lot of the stress you feel at keeping it a secret, and will also educate your classmates on diabetes, which can only be a good thing. If you feel this is too much to begin with, trust just one person with your secret, maybe the boy you mentioned, and see how you feel after this. Hopefully you will feel a lot of relief and maybe start to trust others too. I'm sure you will find that having diabetes makes you a stronger, more caring person, who others will admire and look up to in the future. Perhaps you might consider going on one of our support events for children, this will also help you to cope with your diabetes and share your feelings with others who understand.