Answer: It is not your job to make sure he/she attends school regularly. That's where parents and teachers come in. What you can do is ask about his/her problems or motivation. Get the facts: What seems to be the problem? Are you having trouble getting up in the morning? Are you getting enough sleep? What's it like at home?
Ask simple questions, at an even pace. Don't moralize or intimidate your mentee. Take the time to find out if there are underlying problems interfering with school attendance - or if the problem relates to his/her situation in school itself.
