While I usually agree with PG, any player who believes that they have any kind of a binding Ivy commitment before they end the junior year is wishing and hoping.
While coaches have a feel for their school's admission policies, no coach can predict a decline in grades or not achieving a particular score. (And, since there is no margin for error in any decline, the kid will be left at the alter in a minute.) Some kid posting a comittment to an Ivy before they finish their junior year is doing themselves a disservice - number one, the written rules of the conference are crystal clear - admissions runs through admissions; number two what is the fall back plan when other schools stop recruiting?
Every year, the game of Ivy musical chairs ends with kids left out - thinking that the "offer" received as anything other then a rising senior has meant a successful end to recruiting.
And, as with other conferences, Ivy coaches move. In the past four years, Penn, Brown, Cornell all changed coaches. Good luck with that early "offer."
(In rare cases - e.g., "developmental" cases, a kid will rest assured he get in based on a early offer. Developmental cases are totally outside athletics or regular admissions and usually involve buildings and/or high visibility celebrity children.)