Problem is if older Chromebooks are used for Linux in an educational environment there would be nothing stopping a student from whipping up a bootable USB and dumping another distro (bypassing restrictions). I’m also not sure if there is a enrollment mode equivalent Linux (there may be but not sure).
They could just disable booting from USB drives in the bios and password protect it. They could install something like Fedora Silverblue, or even customize the image used to include whatever modifications they want. Any changes they made to the image would be propagated through autoupdates. Kids wouldn’t have root, so they couldn’t forcibly install a different OS. Of course they could install flatpaks to their home directory, which is probably something administrators would want to prevent, but a knowledgeable student can always find ways to do what they want.
This of course requires schools/districts to hire people to manage that stuff, which could be a problem.