Chromebooks in schools are enrolled in a corporate enrollment, even replacing the (small) SSD won't enable you to run an alternative OS unless you reflash coreboot or get the corporate administrator to release the device.

This manageability that is omnipresent, very hard to bypass and included for free is why Chromebooks are so popular, despite the ridiculously short 3 to 4 year software support lifespan of each Chromebook.

That, and I don't see Apple with any services similar to Classroom [1]

[1]: https://edu.google.com/products/classroom

Also, students aren't always delicate with the equipment, so having a cheap device that can be easily replaced with the least amount of friction possible (most user settings will follow across devices, and if the device is already enrolled then policies are applied automatically)