Im in 8th grade and I already know I want to be a teacher when I'm older. Is that weird?

As you see/discover more and more of the things there are to do to make a living or enjoy your life, you'll changer your mind again and again. THAT'S GOOD.
It makes life more and more exciting.

No, most kids your age 'know' what they want to be when they are older. I myself 'knew' I wanted to be a teacher and a mother from the age of about 10. Then I hit 18 and realised that I really dislike children so it's probably wouldn't have been the best idea.

If you know what you want to be when you grow up and are working towards that goal, then it isn't weird at all. In fact, you are way ahead of nearly all of your peers in that regard.

It's good, but plans change.