Special needs is no excuse for bad behaviour or stealing. While the other girl needs help, she also needs help in understanding why taking (or stealing) someone elses medical kit is wrong. K needs to inject and test so perhaps something can be arranged so the other girl doesn't see what is happening. I am assuming the class does things like biology and/or citizenship?, so perhaps it can be raised in biology lessons and the whole class talk about diabetes and why people have to take tablets or do injections?