Two students named Armistead are in the rolls of the Mordecai school:
Eliza Armistead of Norfolk VA was at the school from the beginning of 1815 to the school's closing in 1818. She may have been an orphan: her tuition was apparently paid by a grandmother, Mrs. Newton, before Mrs. Newton's death in 1816; an uncle took over payments, but he too died in 1818. Eliza Armistead appears frequently in the Mordecai family's correspondence about the school: she was considered quite pretty. She studied astronomy and history in her later years at the school, and won a special medal at the last public examinations. She returned to Norfolk after 1818.
Martha Armistead of Norfolk VA was at the school for one year, 1818. Her account was possibly paid by a George Newton.
Turning to the online family histories, we quickly find confirmation that Eliza Tucker Armistead and Martha Juliana Armistead were sisters, the daughters of Theodorick Armistead (1774?-1812) and Martha Tucker Newton (1780-1810). Theodorick Armistead was commander of the Norfolk Naval Shipyard, 1808-1810.
Eliza Armistead married a Navy man, Alexander Porter Darragh (1789-1831), in 1823. They had two children, Margaret Porter Darragh, and Martha Julian Darragh; Martha died as a newborn. Then Eliza died in late 1826, certainly not more than 25 years old. Her widower Alexander Darragh died in 1831 and was buried on Gibraltar. Their orphaned daughter Margaret married at 18, to a cousin, Thomas Newton.
Martha Armistead married a John Williams of Fairfax and had at least four children: Elizabeth Darragh Williams Sharp (named for her late aunt), Rev. Walter Wheeler Williams (d. 1892), Theodorick Armistead Williams (d. 1890, a bank president), and John Newton Williams (b. 1842). Like many Mordecai alumnae, Martha Armistead had at least one of her sons fight in the Civil War on the side of the Confederacy.