This usually happens because the Chromebook is not connected to the user's WiFi yet. When this happens, it can't receive the message that it has been enabled.
Here are the steps to resolve this:
Click near the time at the bottom right corner of the screen
There should be a WiFi icon near the top left corner of the window that opens and it should say Not connected
Click on Not connected
Choose the correct WiFi network
Enter the password
Click the Connect button
After 10 or 15 seconds, the WiFi should connect
After 10 or 15 more seconds, the Chromebook should reboot and the student should now be able to log in
If the Chromebook does not reboot right away, click Shut down at the bottom left corner of the screen (or click near the time at the bottom right corner of the screen and click the shut down button at the top left corner of the window that opens)
Wait 15 seconds and then turn the Chromebook back on
If your Chromebook still shows as disabled:
Hold the mouse over the WiFi icon at the bottom right
If your Chromebook is connected to WiFi, it will say something like "Connected to <YourWiFiName>, Medium/Strong signal"
If your Chromebook does not show that it is connected, repeat steps 1 thru 10 above
If you have verified that your Chromebook is connected to WiFi and it is still showing that it is disabled:
Please contact your teacher/advisor