I'm not sure why though, because I tried it at my school and it worked...
What exactly are you trying to achieve by trying it at your school?! If you're caught playing around with things like this it can land you in very hot water, regardless of whether it worked or not.
Yes, it's fun to play around and school security isn't always what it should be. But that doesn't mean you should be trying to add users onto school systems without authorisation just to see if it works. These sorts of things are often logged, and if an admin notices a strange user and tracks it back to being created in your account, be prepared for all hell to break loose.