You’d want to image your disk to external media, wipe the Chromebook, set up the dual boot, then restore the image to the partition for Chrome.

That being said, I’m not sure you can do this, and, even if it did work, every time you’re in Mint, as far as the school is concerned, your Chromebook would be off. The Kiosk apps, GoGuardian, what have you aren’t going to run in Linux.