He smiles and instantly turns me on?! to point where i fantasize in lessons about sex with him..help?

He's my science teacher and i've had a little crush on him. recently i fantasize a lot about having sex with him (yuck i know..) just his look or even smallest smile i instantly get turned on. i always stare at his ***** when he happens to sit on top of a table while talking to us. then i start thinking about having sex with him..even sometimes in other subjects i'll fantasize about him..how can i stop this weirdness??????