You could use a hair dryer to heat up your skin before your parents take your temperature.
Then when your parents take your temperature your heat will be up usually enough to get you out of school.
Or you could increase your body temperature by using a heating pad or sit in a hot bath that is hot enough for your body but not enough to cause any burns.
Or take the thermometer and put it under hot water until it reads the high enough temperature but don't get it over 100 F or it could mean that the gig is up and your parents will find out.
Drinking some hot tea or hot chocolate can also cause an increase in your body temperature as well.
Or apply a hot rag to your forehead to help increase your forehead temperature.
I would recommend just heating up the thermometer to get it warm enough to seem like you have a fever but don't get it too hot.