Fake having a stomachache, nausea, or cramps. Other than your word, the only other real symptom for these are vomiting, which you can also fake pretty easily.
After eating a meal, start complaining that you feel sick.
If your parents aren’t looking, stick your finger down your throat (but not too far), and you will start retching but not vomiting. When you feel as though you might actually throw up, quickly remove your fingers. Use this technique sparingly, though; you don't want to hurt yourself.
Have fake vomit ready to complete the effect. Get oatmeal and water, run to the bathroom, put the oatmeal and water in your mouth, then spit it in the toilet and show your parents.
You can also fake a vomiting incident by pouring fake vomit on your floor (or on your bed if you want it to be more believable). In the morning say that you don't remember it and act apologetic to whoever has to clean it up. Be careful with making people clean it up because if they look at it too closely they're might realize that it's not actually vomit.
If you've started having your period, tell your parents you have cramps, or it's that time of the month. Your dad probably won't want to even talk about it, and your mom will understand. Neither of them will be able to disprove it.