Margaret B. Crittenden of Halifax County, NC, attended the school for both 1811 sessions.
Martha Maria Crittenden, also of Halifax County NC, attended the school for the first 1811 session.
The word "Crittendens" appears in the school ledger in September 1813, but I don't know the context of that. There don't seem to be any mentions of them in the family correspondence.
Martha Crittenden (1799-1881) had a long association with Warrenton. From a local history written in the 1920s:
"Mrs. Brownlow was Miss Martha Crittenden of Halifax County. She was a lady of means and had received the best educational advantages of her day. She was a boarder at the Mordecai School in Warrenton when the building burned. At her beautiful home, La Vallee, in Halifax County, she had teachers to conduct the education of her four daughters... Mrs. Brownlow was one of the most remarkable women I ever knew. Her courage, her indefatigable industry, her capacity along all lines, her cheerful amiability, her patient resignation when adversity came, were subjects of the comment and admiration of all her knew her..."
It goes on like that for a while. Not sure how one session at the Mordecai school counts as "the best educational advantages of her day," but that's a pretty typical tone of local histories. Anyway, we learn that in 1817 Martha married the most wonderfully-named Dr. Tippoo Sahib Brownlow of Wilmington (c1794-1879).* The couple lived in Halifax County, where Tippoo was a trustee of a female seminary located on their La Vallee estate. In 1849 or 1850, they moved into Warrenton and bought the corner hotel, "perhaps the best known and best kept hotel in the State," according to the same local history (so take that as likely hyperbole). This is one of their grandsons, James Brownlow Yellowley (1848-1914).
*Sidenote: There seems to have been a fashion for "exotic" names among some wealthy white North Carolinians of this generation. Tippoo Sahib Brownlow was obviously named for Tipu Sultan (1749-99), the sultan of Mysore. Another man from a different family, Hyder Ali Davie (son of the governor of North Carolina) was named for Tipu's father, Hyder Ali, also a sultan of Mysore.
I can find much less about Margaret Crittenden of Halifax County. There's a marriage record that finds her matched with Anthony A. Wyche of Virginia in November 1818. This family chart has his name as Augustus Wyche, and shows them with three daughters (Margaret, Laura, and Augusta). (There's a grave for Laura Wyche Henarie (1827-1915) in a Catholic cemetery in Texas. The bio with her listing says she was born in North Carolina, the daughter of Margaret Crittenden and "Emmett" Wyche. So Mr. Wyche's first name is a bit slippery.)
Margaret may have married twice; Margaret Crittenden Carnal (1800-1871) turns up in a cemetery in Louisiana, with a daughter Augusta Wyche Canfield (1830-1898); if that's the same woman, some sad stories are in that plot. Augusta Wyche Canfield looks to have had three little daughters on the eve of the Civil War; her husband died in battle, and all three girls were gone by 1867 (the youngest lived to be six years old; the other two, even shorter). So, did Margaret go to comfort her youngest daughter, widowed and thrice bereaved, in her last years?
Note that I still have no parents' names for Margaret and Martha, nor indeed do I know for certain that they were sisters.
UPDATE 5/15/13: I have heard from a local historian with an interest in the Crittendens. Margaret and Martha were, indeed, sisters, the daughters of Henry Crittendon of Northhampton County; he died in 1803. Their mother may have died before that date, when the girls were very little. My correspondent also confirms that Margaret married a second time, to a Mr. Carnal (whose first name isn't agreed upon in the various records). Martha Crittenden and Tippoo Sahib Brownlow had four daughters, it seems: Margaret (Mrs. James B. Yellowly); Elizabeth (Mrs. Benjamin W. Edwards); Rebecca (Mrs. Nathaniel Edwards), and Ellen Brownlow (unmarried), who was a teacher in Warren County, and who (many years after the Civil War) was in possession of a breastpin containing a lock of Robert E. Lee's hair.