Sweetie, all young people have crushes on their teachers in school. But you need to realize that it would be highly inappropriate to try to take this any further. I know that it is difficult to deal with when you think about it all the time. You must concentrate on your studies, and other things to take your mind off this teacher. Make freinds your own age, and eventually you will find the right guy who is among your peers who is a more suitable crush. :)
Don't worry, it really is natural all the way. Dream all you want, don't say anything to him or anyone else, enjoy the fantasy and don't act on it. Crushes are pretty nice, what they do is say something about you. If you crush on a nice person, or a powerful one, or a smart one, it just means you want to be approved of by a nice (or powerful or smart) person. Nothing unusual there!
I had a HUGE crush on my 7th grade English teacher :) I changed schools at mid-year and then came back the next year and was totally over it. It is normal. It will pass and you will crush on someone else. Try to focus on your school work.