straight from the AMCAS manual:
"The AAMC disseminates application information to medical schools to which you have applied. The AAMC also shares acceptance information across schools in order to assist medical schoolss adherence to AAMC recommendations. This acceptance data is shared as follows:
a) Beginning in February of each year, a medical school that has accepted an applicant can view the other school or schools that have accepted the applicant, if applicable.
b) Beginning in April of each year, a medical school can view the school or schools that have accepted an applicant, even if that applicant has not yet been accepted by the medical school."
basically, if a school accepts you, it can see where else you're accepted starting in february. by april, any school you've applied to can see where else you've been accepted.