There was a student at the Mordecai school named Maria Crenshaw. I have her attending for the last three sessions (mid-1817 to the end of 1818), from Hendersonville VA, with an adult named Branch Osburn possibly connected to her account. "Branch Osburn for M. Crenshaw" appears in the school ledger for July 1817; and "Osburne for Miss Crenshaw" in November 1817; again the names are paired in the January 1818 ledger page. In June 1818, a "Capt. Knight" is paying for Miss Crenshaw's schooling.
Now, bear with me, this gets a little complicated: Branch Osborne (b. c1793) of Nottoway Co., Virginia married twice. His first wife, Elizabeth Guerrant Dupuy, was the older sister of a Mordecai student, Elvira Dupuy (more on her soon). His second wife, Mildred Carter, was kin to the Crenshaws--her stepmother was born Lucy Anne Crenshaw, and two of her brothers married women named Crenshaw (presumably kin of Lucy's). So we have this Mr. Branch Osborne connected to the Mordecai school through his first wife, and to the Crenshaw family through his second wife. But still no Maria Crenshaw...
So I feel like I'm close, but no bingo. Maria Crenshaw was very likely one of the Crenshaw/Carter/Osborne clan living around Nottoway and Amelia Counties. But which one, and what happened after her brief stay in Warrenton, I still can't say.
(A further tidbit--there are Marthas in this family. Did I misread Maria/Martha? Doesn't seem likely, but maybe.)