I've owned and used the CR-48 prototype Chromebook model, which very well did have a developer mode and a third kernel option built right in. Ran Ubuntu on it with no issues. This has been possible since before the device family was officially available for purchase.

The school thing is different, but also hardly unique. A school issued macbook is often similarly locked down and unusable as a dev machine, due to the student lacking permissions to install anything the school deems dangerous.