Can I tell my teacher I have a crush on them now that I'm graduating?

Please don't. It will just embarrass him. As ? said, he won't be able to date you anyway (if he were to date a former student so soon after graduation, people would think you had been involved before you graduated), so there is really nothing to be gained by telling him you had a crush on him. Furthermore, you deserve more than a booty call for a first relationship, and the age difference, combined with your description of staring at his butt and having sexy fantasies, does not suggest that this is a "love match". The good thing is that since you are graduating, you won't be seeing him regularly anyway, so it is likely you will quickly forget about him.