There is something very wrong with the school (system) if you actually got expelled for that. If that is the whole story, they should have explained why it was

There is something very wrong with the school (system) if you actually got expelled for that. If that is the whole story, they should have explained why it was wrong and tried to encourage you to learn more, responsibly, by actually asking you to help them with securing their system. That is roughly what my headmaster in Russia did in similar circumstances. The thought of expelling a kid over something silly like this wouldn't even cross anyone's mind.