This is a big year for wait listing by NYC private schools.

Last year, ISAAGNY changed the wait list policy to be better for families.   In short, you could continue to remain active on waitlists after the applicable ISAAGNY reply date, even after you had signed a contract with another school, as long as you agreed to be liable for tuition/deposits per the terms of the contract you signed.

This year, ISAAGNY reverted to the original policy.  From their website:

If your child has been placed on a waitlist you may have to call the school to notify them that you want to remain on the waitlist.  If you have not enrolled your child in an ISAAGNY school by the parent reply date, speak directly to those schools where you were on a wait list to find out if they continue to keep this list after the response date.