My guess is that you are beginning to like girls more than you used to and don't want anyone to know. You would feel embarrassed for your class to know. Perhaps they tease about things like this a lot. The reason you washed is because you don't want to feel this way, so you want to "wash it away" and pretend you do not feel this way. You are having feelings that are new and maybe scary,
These feelings are the kind of feelings that are private feelings and not something you want on display for all the world to see. These feelings are not dirty, but they are very personal and you feel sensitive about it.
The girl in your dream was "in some kind of club" probably because you realize girls are so different than boys and you don't always understand them. That is why it feels like they are from another planet of some special group of their own. In some ways, this is very true.
The girl probably does not hate you. In fact, she might even like you a little, or she wouldn't go out of her way to try to prove she does not like you. UNLESS you are mean to her, of course.
If you are being mean to girls to prove you don't like them, it is time to stop. It won't be long until you are wishing you had been nicer so they would think more highly of you. If someone teases you for being nice to girls, just smile and say, "Well, I'm just a nice guy, that's all. This world can use a few more nice guys," or something silly. Then change the subject or start doing your homework.