Oh my goodness! What an awful situation your in!
If your sugars were behaving so erratically whilst you were in hospital, then that is your proof isnt it? If you were forced to let the nursing staff administer all your medication - then that is proof that you are NOT fiddling your insulin surely? Do you have details of all your blood sugars etc that were taken in the hospital?
I cant believe that your consultant is treating you with such contempt - isnt it bad enough that you have diabetes to deal with without accusing you of something you havent done.
And sending the letter to your school must be against the data protection act or the confidential patients rights ( cant think of the actual name of it).
If,as you say, you are completely innocent of doing this then i think your best bet would be to make a formal complaint and ask for an urgent change of consultant and diabetic nurse! I am sure you have a right to do this, without reason, and without being referred by your existing consultant.
Please concentrate on your diabetes and ask your parents to deal with all the paperwork, i am sure they wont mind, and you need to be concentrating on your health issues rather than dealing with idiotic consultants etc..
I hope you find ways of resolving all of these issues and move on with good medical care. Bev x