How can a middle school student get a head start on becoming a science teacher?

look into science academy's at local high schools

Start by taking honors, accelerated, or AP (advanced placement) classes if your school offers them and you are smart enough to take them.  AP classes give you the opportunity to test out of certain college courses, which will get you through college a bit faster.  Don't overdo it though, that will just completely stress you out.  In high school, there may be classes where you can go and shadow a teacher, I know there was a class like this at my high school.  If you want to be a science teacher, make sure you take a good science class every year, even if it isn't required to graduate.  Also, see if there are any opportunities available to tutor students.  This is good practice.  Remember, you're young still, so keep your options open.  By the time you get to college, you may change your mind.  I wanted to be an english teacher in junior high, but now I'm a pre-med student with aspirations to be a psychiatrist.

Good Luck!

Sounds like you're serious! Okay! Well, as far as your schooling goes--take ALL the science classes you can. This also goes for math classes. They go together. When you get to high school, continue this trend. Hopefully, that will include physics and calculus classes.

Over the summers, see if you can get into summer science camps--and eventually even work at these camps as a counselor in the summers. Also, get buddy-buddy with your science teachers (after they get to know you and your work ethic) and ask them if you can TA for them (my school only allows juniors/seniors to TA for teachers but it's incredibly helpful). Also ask if they know of any programs you can get into or people that you should meet.

Eventually, you're going to want to observe science teachers in the classroom after you leave high school. It's hard to see the other side (the teaching side) and fully understand it until you leave your high school life/world behind (check out both middle and high school classes). Look into colleges and universities that have strong education and science departments. If they do their job to help you, you'll be in and out of college in four years--and with two degrees.

I do want to note that it's not a bad idea to keep your mind open to other things you love. Take fun classes in high school (while they're free to you and see what else interests you. Even if you're set on teaching science, you're going to need a break every so often. So take some time to find other things you enjoy. Good luck. Science rocks.

Note: When you go to observe science classes, go to different schools. Each school (and district) is run a little differently. Get a feel for it. You'll have an even better idea of what you like when you student teach. Science is going to be a big need area very soon, so when you're ready, you could have some great choices to choose from.