The reason is very simple, you already know what the ins and outs of your school network are, you also know where the weakness are. You've tried to warn them about these security issues, but they have been negligent you take the matter into your own hands and decide to do a bit of poking around on the network only find that you managed to break into your schools principal computer.
Now they find out it was you who did it, and they give you a warning stating that you shall not attempt to do any illegal activity ever again or face expulsion. Months passes and nothing has been done about the security breach, they haven't even patched the security hole.
Now from this we can learn that, if you were a bad student and got expelled from school you could from outside hack into the schools system and still break into the principals computer.
Moreover that's what they mean by this, there could be other reasons too but that's the only reason I can think for the moment.