Hoodlum pretty much said it but I thought I'd add in a few details:
1. Taking Basic Soaring (different from AM-250, which is the one everybody takes as a 4*) during your 3* summer is indeed a mandatory prerequisite if you are interested in continuing on to become a Soaring Instructor Pilot (IP).
2. At the end of a particular 3-week period of summer soaring, program staff conduct interviews of all applicants interested in upgrading to become IP's. Approximately ~50 cadets are selected at the beginning of the fall semester, and spend the next year upgrading to IP status (6 hours at the airfield every other day, sometimes including Saturdays). At the start of the spring semester, eligible non-selects are given an opportunity to re-apply, and about ~25 more cadets are added to the program. These "spring upgraders" complete the same program, but only have approximately 1 semester in which to do so, meaning that their completion is significantly more accelerated (including a spring break flying trip and more Saturday fly days).
3. There are also two Advanced Soaring teams (Aerobatic Demo and Cross-Country), which each select ~6 of the upgrade cadets to join the team during spring of each year. From my understanding, these members are typically "fall upgraders" due to the requirement that candidates have completed the baseline IP upgrade program prior to beginning specialty training. Airfield time commitments are significantly higher for those on Advanced Soaring teams due to additional training/practice requirements.
Hope this helps. Feel free to PM me if you have more questions about Academy soaring or the IP program.