When I was a kid, I remember sticking the thermometer on a light bulb trying to fool my mother into letting me stay home. Then I burned my tongue when I tried to stick the thermometer back in my mouth. Ha ha! I must have passed that on to my kids, because they always used to try and tell me they were too sick to go to school. But I finally learned how to stop that problem and here is how; I instituted a rule that if any child is sick, they must stay in their bed without anything but books to entertain them for the entire time they are home. No T.V., no video games, no iphones....nothing....never leaving their bed.
This rule had cured my children of most ailments, it's quite the miracle! But before you are too hard on your kids, be sure to check them for a fever, and review their symptoms to be sure you are not making them go to school sick. You don't want to pass the illness on to others. I carefully watch them when I tell them my "rule". If suddenly they are peppy and ready to go, then I send them to school. But if they are listless and say they don't care if they stay in bed all day, I usually let them stay home. It's a good way for me to tell if they are faking. I hope it works for you too!