While "offers" from IVY's may be coming in earlier, admissions has the ONLY real say. A COMPLETED application (including LORs, scores, guidance counselor letter, etc.) will be considered by specially constituted admissions committees from roughly early September senior year. The more experienced the HC, the better the feel he has for the admissions bar.
I personally know of players who received Ivy "offers" who were only to learn that the "offers" couldn't be consummated.
With schools with lower admissions requirements, there is a lot more margin for error in case of a hiccup in scores or grades. There is no margin for error for IVY applicants. (For example, I know kids who were repeating tests into the middle of their senior year to get their offering schools' required minimums. No IVY coach will wait for that to happen and risk losing one of his 6 - 8 recruits.)
Obviously, once the kid hits his IVY "number" (28 for All-American potential; 31 team average at Yale several years ago) that element is done; and a kid who had straight "As" in the most rigorous cirriculum for the first two years of HS is not a real risk of plummeting academically (e.g., the kid who has finished AP Calc A, earned an "A" and got a 4 or 5 on the AP test as a sophomore). That kid isn't at risk.
But the kid who got a 28 (not AA potential) is at risk; as is the kid with a 3.5 GPA.
Moral of this story for IVY hopefuls: test early (maybe end of Soph year or first test Junior year at the latest) after thorough test prep, look well ahead (understand before ninth grade what "the most rigorous cirriculum" means for your school), be aware daily of your kids academic performance (e.g., EDLINE type programs), address issues early (e.g., tutoring if needed), understand testing schedules (AP, SAT, ACT) and their interaction with each other and baseball schedules.
The rewards waiting for those 50+ kids (the total recruiting class of the entire league) are impressive - but it requires a family team effort to put the puzzle together.