How can you start being called by a nickname?

I had this same problem but I came up with a perfect solution and now everyone calls me by my nickname Cat! Even my teachers!

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Step 1: Wait until the next school year when you have a new class

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Step 2: When you introduce yourself, say "hi my names ____ but everyone calls me ____.

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Step 3: Sooner or later everyone will call you by the nickname!

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(It doesn't matter if people actually called you by that before, no one will know and you just created your own nickname! But be careful and choose one you will like for a long time! Nicknames stick around!)