well something which keeps copying it self would be called a "worm" . Make an auto execute usb which would require you to create "autorun" file , I actually made a Blog post Explaining:Autorun.INF/AutoPlay & Downadup USB Worm about 9 years back, maybe you can get an idea and create one ..
Please note it would be unethical and illegal to create something like this and it might get you kicked out of your school if caught , so you may make it and test it for research and learning phase but not otherwise.
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