There are two ways. First, because your main organs haven't changed, you still attend as a boy, but you should pay attention to cover up those feminine features in dressing and makeup. You can use illness as an excuse to cover up your different performance, leave the party early and avoid exposing flaws. The second is that someone will know that you are a transgender anyway. This time, it is better to be generous and plain, so that students can be psychologically prepared. If you don't want these two methods, you can only lie and refuse with some excuses that you really can't come.