Don't tamper with hardware that somebody else owns. If you get caught, you could be fined a lot more than 50€ and expelled. School administrators often like to "make an example" of kids that they think are "hackers" even if you're just booting Linux from USB. They don't understand the difference between that and real hacking, so don't risk it.
You can only achieve true privacy on hardware that you own. A cheap laptop to boot Linux isn't a bad idea.