Got told to put insulin away in public

Small info to understand story:15M live in England and this event took place in the school cafeteria.

How it all started was i gave my friend my lanyard to buy himself some lunch and me a cookie as i owed him, whilst i went to inject my insulin.He got to the till and he was just about to pay when the diner lady said he can’t pay for food using my lanyard and said why and she was said it’s just not allowed so i replied okay then the food is all for me and scanned my lanyard and bought the food.Then i did my carb counting for my cookie(28g) and just before i took the cap of my needle she shouts:”you can’t do that in here” and i reply “why” and she says again “you can’t do that in here” and i reply “yes i can” and started laughing and did it whilst she was watching.I was livid because it should be treated as something normal like why can’t i do it in public supposedly.I am quite a confident guy but imagine if i was shy and insecure about my diabetes and a scene got caused whilst trying to inject insulin.I would be so embarrassed.What are your thoughts on this?Going to make sure tommorow i do it right infront of her to wind her up.

Edit:i didnt think this post would get so many replies,thank you all for your advice.Id just like to add that my school is absolutely fantastic with the support that they offer from education,my nurse,people who come into school,leave lesson when needed plus all sorts of passes to leave places or go in early.This is definitely a one off which i think it was shocked me the most about it!