If you see a message that says Backup Delayed, it means that the most recent backup attempt did not complete successfully. If you plug in your drive and let it complete that message will go away. As long as you see a date or time when you click the Time Machine icon, you will know that you have at least one valid back up. If it says 'Back up device not configured' or you are having any other trouble with the back up process, bring your computer and back up drive to the Tech Center where someone will help you trouble shoot the problem.
If you cannot find your back up drive your family is responsible for purchasing a replacement through the school or on your own. The back up drive should be USB 3.0 and have at least 500 GB of storage space.