My 6yo daughter has a name that has a shortening that I personally don't like, it was one of the weighing factors to giving her this name but we decided if we stuck to calling her by her name until she was old enough to decide for herself what she would be called the others would and that has worked. My daughter also hates this shortening! Not because we've said anything to her, she just loves her full name (the whole thing, middle names and surname too). Her dad recently realised she hates it and will call it her if he's trying to play her up, which works, she'll start trying to beat him up.
At school there are two teachers who call it her and when they do she breathes in deeply through her nose whilst pushing her lips out. I've asked her why she doesn't say anything about it but she won't, it's not in her nature. I was talking to a senior member of staff recently and mentioned it, she said she'd noticed she hated it too and would make sure people know not to call it her.
About a week later she comes home from school and says "[TA] said she has to call me... because my name is so long and she has lots of children" her name is 6 letters long and "shortening" it makes it 5 letters and not much quicker to say. I'm not sure if this is in response to the senior teacher saying something or she's stood up for herself, but either way if someone doesn't like being called a name, isn't it rude to tell them their name is too long and they have other people's names to think about?