If your overall average drops below 80 at Seniors, they will board you starting week 3 or so. You will probably get some kind of probation at that point. I don't think they'll let you graduate with an average below 80 (or maybe a little less, it is at the Colonel's discretion.)
The mission of Jr.s is to "educate and motivate", thus the moto movies on Sunday nights and the visit to Washington. At Jr.s they will keep you on unless you DOR, NPQ or are an integrity violator. They want you to make mistakes, learn from them, and come back to Seniors ready to kick ass and take names. The mission of Seniors is "to train, screen, and evaluate" and there is much less of a tolerance for failure. If you have a 65 overall average at the end of week 4, you will be going home.