I agree Hannah, that is outrageous-she shouldn't have spoken to you like that under those circumstances. No one likes to be made to feel different in school, and she should have been more sensitive.
I presume the school are aware of your diabetes. If they are, your individual teachers should also be informed about it, and know what to do if you had a hypo in class or became unwell. You should speak to your Head of Year about your drama teacher's reaction and how it upset you. If this teacher was concerned about health and safety, regarding your injecting in class, this should have been discussed with you.
There are always going to be situations where you need to inject in public, on buses, restaurants,etc - most people do this without reactions like the one you got from your drama teacher.
Don't dwell too much on it, everyone makes mistakes, even teachers-I'm sure if you or your head of Year have a chat with her, she'll be more understanding.
When my son was in secondary school he was allowed to wear a baseball cap, as he had hair loss from cancer treatment. A teacher he didn't know stopped him in the school corridor one day shouting" why are you wearing that cap? You know you're not allowed to wear hats in shcool". My son's reply left this teacher a bit shocked, which he found amusing
It's a struggle enough for you to cope with diabetes at school, you can do without negativity from other people. You should feel proud of yourself for being able to manage it. Have a chat with your Head of Year or Form Tutor.
Jus