This means the Chromebook either has been unenrolled or isn't aware that it is already enrolled. You can often resolve this issue by performing a factory reset on the Chromebooks. If you perform a factory reset and the Chromebook still does not indicates that it is "Managed by desalesmedia.org.", then connect the Chromebook to either Wi-Fi or cellular and reboot again. Try to reboot a few more times - the vast majority of the time, the Chromebook will "recognize" that it is already enrolled and it will indicate that it is "managed" and then you can login with any user account from your school's domain.
If the login screen still does not indicate "Managed by desalesmedia.org.", then you will need to follow the procedure to have it repaired or replaced.
The image below shows what the Chromebook login screen should look like.
The image below shows what the Chromebook login screen will look like when the Chromebook has not been Enterprise Enrolled.
If you see this screen, click More Options and choose Enterprise Enrollment.