I'm assuming you mean a Junior in high school, if so, you're probably talking about the Split Option program. In this program, you can do Basic Training the summer between your junior and senior year, and once you graduate, you'll go off to Advanced Individual Training then into the real Army. Some jobs in the Army however do not split MOS into Basic and AIT, and one of those is Infantry. They have what is called One Station Unit Training (mash Basic and Advanced together), consisting of 15 weeks for Basic and Advanced training. I'm not sure exactly when they cut it off and let you go back home for school, but I do know that they do it.
Seeing as you're interested in doing the National Guard, I would *highly* advise you take a look at their forums as there is a wealth of information for you to take in, ranging from Basic Training, to deployments, Recruit Sustainment Program(which you WOULD attend as a Split-Op soldier), etc.
Oh and Infantry Basic is always at Fort Benning- Home of the Infantry.
http://www.1800goguard.com/forums/index.php