what you are referring to is called the "split option" program. but you have a few misconceptions. yes you can join when you are 17 yrs old and a junior into the reserves. you will go to basic training between your junior and senior year. once you get back you will continue to train with your unit during the training weekends. then after your senior year you will attend AIT (Advanced Indivdual Training). afterwards you will return home and either find civilan employment or begin school as you will still be in the reserves. if you wish to go active duty at this time, YOU MUST inform your chain of command and IF they release you from the unit then you can go active.
you can do the same thing in the guard, however, i have been in recruiting for almost 3 years now. in that time i have had 17 guard soldiers come by my office and request active duty. to this date ZERO have been released to do so. whereas i have moved several reserve soldiers to active duty.