I'm not familiar with IL, but I got my basic while I was in high school. Mind you I was about a year older than everyone in my grade, so I was 18 when I got it. I would strongly encourage you to try to get your EMT, especially if you want to apply for nursing school or do something health sciences related in college.
BUT I have a few caveats. 1) EMS is demanding, and it might be hard for you to find a place that will hire an 18 yo EMT because of insurance issues. This goes without even mentioning the maturity you would need to demonstrate. 2) If you cannot find work, you might not be able to re-certify without experience, so you'd loose your cert. 3) Don't get all high strung if/when you get your cert.
Now, to answer your question: Many EMT programs will overlook their GED/Diploma requirement if you ask for permission and demonstrate maturity and good grades. If you are not 18, you will probably not be able to take the national registry exam to certify - so you'd probably want to think twice before you take the course. There are some fast track EMT programs, but you will have to find them. Use google for that. Just a warning, they are usually not cheap ($750-$1,300).
Most community/city colleges offer EMT courses if you really need a place to start looking. It is almost August, and most EMT classes (even the fast track ones) are around a month long. So, you are probably out of luck finding something that will end before September, let alone finding something that still has openings. If this is something you really want to pursue, you could see if there are evening EMT courses offered at your local community college you could do after school.