Actually in your case our school has a BYOD program (bring your own device) in which you can bring your own laptop with whatever flavor of OS. Firewall would restrict you, your device would be considered untrusted, and in testing a loaner locked down chromebook would be provided. The issue comes with non BYOD devices.

Now lets assume a school has 1k students. If they allowed os unlocking and allowed students to tinker with the os. Then they would need 2k chromebooks 1k unlockable 1k locked down for exam administration (assume the whole school needs to take it at the same time). From a admin/IT perspective why should the school need to pay double the number of chrome books just for a few students to install their favorite brand of linux.

Even under the best circumstances where support queries aren't increased (from students softbricking/ not knowing how to use linux) and say they are able to preserve 1k unlockable chromebooks, admins would still need to replace the other 1k locked down chrome books at end of software to stay in compliance with testing software (negating any financial benefit).