There's a student on the rolls for the Mordecai school named Sally Cary. She was at the school for both 1811 sessions. I don't have a hometown or an adult's name attached to the account, and I don't think she got mentioned in any Mordecai correspondence.
Neither part of the name is particularly uncommon. She's obviously not this Sally Cary, but she could be from that Virginia family--as we have seen over and over here, families tended to recycle first-last name combinations across branches and generations. I do find a Sallie Cary (1806-1848), daughter of Captain Miles Cary of Norfolk County, Virginia. She'd be on the young side of the right age, but without more details, I can't say she's the Mordecai student.