They are all taking advantage of me ?

People take advantage of you because you let them. When she told you that she invited more people, you should have said that she needed to uninvite them. You mention that you are in high school, yet you are planning a party where other teens will be drinking. Now you have more people showing up. What are the odds one of your neighbors will call the cops? Or if you are having it elsewhere that you will get caught anyway. You should have cancelled the whole mess and rescheduled it so you could invite a few close friends.