What's the cost of data insecurity[...]
Use a text file and never save to the hard drives. Bring a flash drive with portable apps on it so you don't have to use insecure apps.
[...]of giving up freedoms[...]
You have the freedom not to go to the school, and thus not use the operating system if you like.
[...]and of supporting a criminal corporation?
I'm all for due process of the law. Will they violate the laws? Probably yes. But I as a consumer won't support them. I'll use Windows if it's given to me as a service, but I won't support them directly.
The bottom line is that freedom will probably cost something. If you have exotic values such as to never use proprietary standards, then you are going to have to make some sacrifices. Most people just don't give a shit. I use Windows at my college and Linux at home because I don't have enough money to grab Windows, nor is Windows actually a priority for me to own.