Try making a Ubuntu USB stick with another PC and make it bootable with something like Rufus. After that, plug it in to the Asus and shut the chrome book down. Now, press POWER and start pressing F12 right away.
It should boot to the Ubuntu boot loader.
If this is a Chromebook you bought: Install Ubuntu and erase the HDD
If this is a school issued device: Use the Try Ubuntu option.
If they somehow locked the boot Menu, I don’t know what else you can do other than replace the OS