Echoing Hammer here, but still -
I wasn't working impressive jobs when I was selected - Working very part time as a personal care assistant, as a rock climbing coach, and as a chef - thought that would be marked against me on my application. Surprise was, my recruiter was more interested in the fact that I managed to piece together three low paying jobs into something that had kept me debt free then in being a general manager. Don't get me wrong, I'm sure that would have been a plus, but there are many ways to demonstrate yourself suitable for a job in the military (or at least satisfy the recruiter)
I assume you are young (please correct me if I am wrong) - I think they realize that younger individuals haven't had the opportunity to have extensive work experience, etc. They want to see that you've taken the opportunities available to you to further your own knowledge and for examples of your work ethic. Your school marks would be one of those strong tangibles they will look at, as well as having well rounded experiences from which to draw experience from (playing sports, hobbies, educational pursuits, etc). Aim for excellence and it will speak for itself.
There are many other threads on this board regarding what makes a suitable (and exceptional) candidate - settle in here for a while, fill out your profile, read EVERYTHING on here you can find (I've learned more then I expected simply by going though the boards and reading), and reap the knowledge that is at your fingertips. One of the things they look for is the ability to take the initiative and accomplish your tasks through your own hard work - no better place to start then now.
Good luck! if you had any particular questions about the recruitment process, I'd be happy to answer them if you send me a personal message (PM).