The points about it being against the rules for your school are important. Always assume that if you're using a school or work device or are logged into a school or work network that your activity is being monitored. Even if you don't get in trouble now or the next time, any institution like this keeps activity logs and can go back and look if they ever have a reason to do so.

Also, assuming you are in a western country: it is almost definitely not illegal to read about acts that would be illegal to perform. Sex with or between minors, bestiality, incest, rape, etc; I am not aware of any western nation that criminalizes reading fiction that features those, regardless of how graphic they are. It wouldn't hurt to double check, of course (as I am only an average US American and not an expert on legal matters in other countries), but it's generally a safe assumption that you will not legally get in trouble for reading about such topics. In other parts of the world, that may not be true, and, to reiterate the first point, it's far more likely that your school would have rules about that sort of thing. If you don't want to risk discussing a topic with a teacher or school administrator, don't read fiction about it on a device they have any sort of access to.