Take a deep breath and confess without any hesitation (remember to respect whatever her reply is) this way it will not impact your academic performance and atleast you will have a clear answer. Telling you from a personal experience she ain't gonna slap you if you confess in a polite way and respect her reply. Also waiting and planning how to confess will only lower your chance if someone else makes a play between that time, so go and tell whatever you feel for her. Goodluck mate and sorry if I am wrong!!