How do I give a gay vibe for others to know I'm gay if I want guys to approach me at school?

You don't really want to do this in school because it will be easy for bullys to get together and make your life miserable. I know it does not happen as much these days but it can and it would be awful for you. It is unlikely that other gay guys will approach you in school anyway. They would much prefer to do any meeting outside school and more likely in private at this age. There is plenty of time for you so please wait until you have left. You may want to join a club or socialize in places where gays go when you are a bit older, this is where most find friends.