I think that taking the position as a junior EMT will show that you know what you are getting into: the hectic pace of medicine, the emotional turmoil facing the families of accident victims, the blood, etc. So, in that regard, I think it is good. I don't see how it could hurt!
What I am concerned about though is how advanced and how rigorous your HS education has been. I graduated early, and I missed out on a lot. HS helps to prepare you for college. The more prepared you are, the better your grades will be, and the more favorably a med school will look upon your application. I would not be in a hurry to finish HS - take full advantage of everything it has to offer. Your college classes and grades will be much more important to your admission into med school than will your experience as a junior EMT. If you can find a way to do both, that is fine, but do not let your education suffer because you want to do the EMT job.
Now, if you are not totally certain about medical school, you can explore a career as a Paramedic. It's a lot less schooling, and the pay is pretty good. Or, some other medical field: radiology, etc. There are many professional medical positions that you can do with 2-4 years of college, rather than going on to medical school after earning your degree. You will find a list of all careers in the Health Science field at this link, with hyperlinks to additional information about each of them.