But why you don't want to go to a FRIEND'S party. If really a friend, then cherish the opportunity. If really can't go, then perhaps you have a good excuse that you have to share honestly. If you're upset with that friend, go but later discuss it with that friend. And if nit really a friend, then don't call him/her a friend. You can just tell'm you couldn't go!
if you dont live with your parents or dont want your parents to sound lame just say that youve been feeling crappy. dont make up some illness, just say youve been a little too tired, having some bad cramps (will work bets for girls), or coming down with a cold. never make it too severe and keep it simple.