How long do medicines suppress fevers for your kids? Does this differ kid to kid?

My kids are sick and a co-worker was confessing to me that she sends her kids to school with fevers (yuck). She doses them right before school and by the time the fever comes back it's almost time to pick them up. Ignoring how rude that is to other people's kids I can't understand how that works time wise.

If medicine works on my kids' fevers their fever is back at almost exactly 3 hours with Tylenol and 4 hours with Advil. I can practically tell time based on it. By that time they are hot mopey puddles again. The effects of the medicine never lasts as long as the time window given for the dosing instructions. There is no way they would get through a morning let alone a school day (not that I would try it). I questioned my coworker about this aspect of it and she looked at me like I had three heads. Ibuprofen worked for 7 hours for her kid every time.

Does it vary that much kid to kid?