Okay, I'm not a doctor so I could be wrong, but I'm pretty sure you don't need to be off school for months if you have mono as a teenager. Usually it's no more than a couple of weeks spread out if you have the longer annoying type that can last (I had this and still managed to get to school for at least brief periods of time). But you can still very easily go to school and not be infectious to others. If you have it for a short time, then you're not gone for long at all.

As for fatigue, in a young person, should not be bad enough as a reason to be at home and not in school.